# AP Physics 1 - Unit 8 - Live Kahoot! Multiple-Choice with Flipping Physics

## Метаданные

- **Канал:** Flipping Physics
- **YouTube:** https://www.youtube.com/watch?v=Ffh1OwT2a9k
- **Дата:** 22.04.2026
- **Длительность:** 1:13:22
- **Просмотры:** 1,847

## Описание

Join Mr. P for a live AP Physics 1 Kahoot! Test your knowledge with a 10-question multiple-choice quiz covering Unit 8: Fluids. Challenge yourself, compete with friends, and sharpen your physics skills — all in roughly an hour!
📅 Tuesday, April 21 at 7PM ET
Want more practice? All of these multiple-choice questions and much more are available in the Ultimate Exam Slayer:
👉 https://www.ultimatereviewpacket.com/courses/physics1-slayer
#apphysics1 #unit8 #fluids

## Содержание

### [0:00](https://www.youtube.com/watch?v=Ffh1OwT2a9k) Segment 1 (00:00 - 05:00)

Flipping [singing] Physics Good morning, everyone, and welcome to my Unit 8 AP Physics 1 live Kahoot multiple-choice quiz. I believe we are just We have just over a fortnight until the AP exam. Or is it 3 weeks? No, it's 3 weeks. No, it's two. It's 2 weeks. Holy cow. Anyway, hi. Welcome. So, today, obviously, please sign up for the Kahoot. You can see the pin and everything right there. So, yeah, I'll be talking while you sign up for the Kahoot. Thank you so much for that. So, be aware that um I'm having a 14-multiple-choice all AP Physics 1 Kahoot for YouTube again a YouTube live on Tuesday, April 28th at 7:00 p. m. Eastern Time, which is 1 week from today. That's right, We're going to do a review of all the topics. We have 14-multiple-choice questions. Today, we're only doing 10. And then 2 weeks from today, yeah, so it's a fortnight and a day away the exam. So, 2 weeks from today, I'm doing another 14-multiple-choice all AP Physics 1 Kahoot uh which is going to be exclusively for my Ultimate Exam Slayer students. So, that's on Tuesday, May 5th at 7:00 p. m. Eastern Time. So, that again, 2 weeks from today or 1 week from today, we've got a review that's for everybody. And 2 weeks from today, we have a review that is exclusively for my Ultimate Exam Slayer students. But, all are on Tuesdays at 7:00 p. m. All right. Um so, section 1 of the AP Physics exams have 40 multiple-choice questions in 80 minutes. So, that is 2 minutes per multiple-choice question. So, that's what we're going to do today. It's going to model what you're going to feel on the AP exam. You'll have 2 minutes per question, roughly. Clearly, on the AP exam, you'll have uh um 40 questions spread out over 80 minutes. So, you'll have, you know, different time on different questions. But, today, we're just going to model what it means to have 2 minutes per multiple-choice question. So, take a moment to say hello to our moderator, Ms. Bonk. She is in the chat helping out with tech issues and maybe some physics issues as well. Um so, we'll see. But, say definitely say hello to Ms. Bonk. She is an AP Physics and Project Lead the Way teacher at Addison Trail High School uh in the suburb outside of Chicago. I do have to mention, of course, my Ultimate Exam Slayer Flipping Physics and my Ultimate Review Packet. We are, as we talked about, just over a fortnight away from the exam. So, I definitely recommend getting my Ultimate Exam Slayer and or my Ultimate Review Packet. I, of course, have all sorts of free stuff on Flipping Physics. I have all of my review videos. I have some uh exam solutions and things like that um on my website, which you can certainly find at Flipping Physics. But, I also have the Ultimate Exam Slayer, which is specifically designed to help you understand the exam itself. So, there's all sorts of stuff in there about the exam structure. I've gone through and identified different multiple-choice categories. I talk about the different types of free-response questions. I have uh multiple-choice quizzes for every exam. I have two practice exams. The Ultimate Review Packet is more designed to help you understand the curriculum itself. So, it's for the con- the Ultimate Review Packet is for the content. It has all sorts of stuff sort of stuff in there uh study guides um stumbling block videos, things like that, and one practice exam. And the Ultimate Exam Slayer is more for understanding the exam itself and studying for the exam. All right. Uh so, here we go. I think — Cast Lucky, I'm going to put that up there. I'll even talk about it for a second. So, Cast Lucky, "Hello, are the Kahoot bugs fixed? " So, I figured it out. I'll talk cuz we still have people signing up. So, I'll talk briefly. So, last week, we had an issue, which is hopefully fixed this week. Uh so, it turns out I'm pretty sure this is what happened. So, the default type of Kahoot switched from classic to professional, and I didn't realize that. So, I had chosen a professional type of Kahoot, which doesn't do the leaderboard and things like that. So, I've chosen classic this time, and we should be okay. There should be a leaderboard. Everything should be okay. So, just be aware that. Um So, we're almost there. So, I'm going to take a moment to take I have to rearrange stuff to make it so that I can start the Kahoot. So, give me a few moments. I'm going to do that. So, this. Then, I do that and let me think. Um that. Move this over here. Let me just check. So, I've got this one, which has, yep, all that. And then, I have this one, which has the Kahoot.

### [5:00](https://www.youtube.com/watch?v=Ffh1OwT2a9k&t=300s) Segment 2 (05:00 - 10:00)

Uh [sighs] so, it looks like we're pretty close. A couple people are still signing up. What happened there? That's a weird artifact. Whoa. I don't like that. Hold up. I got to fix that. That's a weird artifact that's happening there. What on Earth? Okay. Hopefully, that stuff will show up. Do do. Now, it is showing up. Is it going to be here? Sorry, I'm trying to figure out why there's that black line. Okay, I think we're good. Okay, so, here we go. We've got But, I want that on the other side. So, I want to move this one over here. All right. That's a fun scree- — That's a fun screen grab. Yeah, that's fun. Okay. Um Welcome back. Okay. So, I think we're going to get started. So, in order to get started, um actually, no. I see people are still signing up. So, I'll take a few moments. Take a water break. All right, here we go. Um I am going to get started. So, let me just make sure that over here, we're going to have, yep, so there's going to be the first question. And I am going to press start so that we can get started. Uh people are 6 minutes late, so here we are. We're getting started. Boom. Kahoot 360. It's going to give you a whole Flipping Physics. This is brought to you by and then, Unit 8 AP Physics 1. And you're going to have 2 minutes to answer this question. All right. So, choice B is the correct answer, and we got a lot of people who answered choice B. So, let's figure out why choice B is the correct answer. So, we have our question, which is right there. Let's make it a little bit bigger. And here we go. Question 1. So, we have the buoyant force equation on the exam reference sheet is capital F sub lowercase B is equal to rho times capital V times lowercase G. Class, what subscript is missing from this equation twice, and what does that subscript mean? Now, anytime I say class, I'm referring to you, my class right now, and I'm "Please answer in the comments and answer correctly if you can, please. " So, class, what subscript

### [10:00](https://www.youtube.com/watch?v=Ffh1OwT2a9k&t=600s) Segment 3 (10:00 - 15:00)

is missing from this equation and what does the subscript What does that subscript mean? And if that subscript is missing twice. All right, here we go. This is fun. We have some answers here. So, first off, um the object is not correct. V displaced is not correct. D displaced initial is not correct. This is kind of fun. Um — F for fluid density and volume of displaced fluid. Okay. So, oh, even better. We've got uh F is for fluid. So, we are so close here, but it's a very important to understand what is so it is you are correct that it is an F that is missing twice, right? So, it is that lowercase F is missing off of both the density and the volume. So, this is the density of the fluid displaced by the object and the volume object. Those two subscripts are so important. I don't not know why they're left off of the exam reference sheet, but please remember that both of those have refer to the fluid displaced by the object. So, it's the density of the fluid displaced by the object and the volume of the fluid displaced by the object. Okay. So, all three objects displace water, so the density of the fluid displaced by all the objects is the same. So, it all comes down to the volume of the fluid displaced by the object. So, all three objects are submerged. That means the volume of the fluid displaced by the object is equal to the volume of the object for all three objects in this particular example. Again, because all three objects are submerged. So, objects one and two are the same size, therefore both objects one and two displace the same amount of fluid, therefore the buoyant force acting on object one and two are the same. There is only one answer choice that fits that, so the correct answer is B. You don't even have to, in order to figure out the correct answer choice, uh figure out what happens with three, right? So, we're going to do that, but realize, obviously, on a multiple-choice quiz, you are um rushed for time. So, you could actually stop there and move on. You might want to flag it if you do have time to come back, but you could move on. So, here we go. Object three has a smaller volume, therefore the volume of the fluid displaced by object three is less than the volume of the fluid displaced by the other two objects. So, the buoyant force acting on object three is less than the buoyant force acting on the other two objects, therefore the correct answer is still B. Great. Uh moving on to question two, but of course, before we move on to question two, we're going to check That's right. the leaderboard. All right. Aquatic Hedgehog. Congratulations, Aquatic Hedgehog. Let's see how long you can hold at the first place. Okay, here we go. So, moving on to question two. Question two looks like this and I will make it a little bit bigger so you can read all the answer choices. Are a little smaller. Yes, do do. Boop. There we go. — [snorts]

### [15:00](https://www.youtube.com/watch?v=Ffh1OwT2a9k&t=900s) Segment 4 (15:00 - 20:00)

— All right, so answer choice A is the correct one and a lot of people answered that. That's awesome. Let's figure out why answer choice A is correct. Here we go. So, contact pressure for an object on a surface is pressure equals the force perpendicular to the surface divided by the contact surface area. So, both cubes are the same size. Both cubes have the same contact area with their surface. So, that uh part on the bottom is going to be the same. However, the force of gravity acts straight down on both objects, right? So, that force perpendicular for one is just equal to the force of gravity, so the pressure for force one is just the force of gravity divided by the contact area, whereas the force perpendicular for object two is equal to the force of gravity perpendicular. So, pressure two, pressure on object two, is that force of gravity perpendicular divided by the contact area, which so it's going to be this right here, that force of gravity perpendicular, the component of the force of gravity which is perpendicular to the surface. So, the force of gravity, by definition, is greater than the force of gravity perpendicular and both areas are the same, therefore the pressure caused by one is greater than two. So, the correct answer is A. Great. Moving on to question three, but of course, before we move on to question three, let's see how we are doing. Ooh, Brilliant Elephant taking the lead. Brilliant Elephant, good job. All right, so let's see. Let's move on to question three. Question three looks like this. The whole thing fits, so we'll move on. All right, the correct answer is choice D. And again, people are doing great. This is awesome. I do want to answer address a comment somebody made. So, somebody said the reference sheet for the AP exam has a key that tells you what each notation is. Now, this is true, but you need to realize that the reference sheet for the exam, that key, doesn't actually have all of the letters on it that are used, and some of the letters, for example, H is used in different times and means different things and it doesn't identify that on the reference sheet. So, you really should familiarize yourself with that reference sheet so you understand what everything means and have looked through the key and everything because again, the key, the legend doesn't actually have all of the information on it, which is frustrating

### [20:00](https://www.youtube.com/watch?v=Ffh1OwT2a9k&t=1200s) Segment 5 (20:00 - 25:00)

but that's the way it is. All right. So, we're going to figure out why choice D is correct for question three. So, here we go. Question three, that's going to be big enough. Okay. So, question three, pressure in a reservoir is absolute pressure in a fluid. So, that equation is the absolute pressure equals P not, the pressure at the very top of the fluid plus rho gh, the density of the fluid times the gravitational field strength times the depth of the fluid. So, I'm going to rearrange that slightly. So, we have the absolute pressure equals parenthetically, density times gravitational the fluid density times gravitational field strength uh that times the depth plus the pressure at the top, P not. So, I'm also going to put Y equals slope times X plus B. I don't use M for slope because M can get very confusing in a physics class. So, Y equals slope times X plus B. From that, you should be able to tell that B is the Y intercept, which is pressure not the pressure at the very top. So, you can actually see that the pressure at the very top is 1,000 pascals. So, you can actually see that from just the graph here. And density times gravitational field strength equals the slope of the line. So, we need to pick two points on the line, determine the slope of the line, and set that slope equal to the density of the fluid times gravitational field strength. So, we're going to pick point one as right here of 0 m and 1,000 100,000 pascals, excuse me. And we're going to pick point two to be right here, which is 8 m and 200,000 pascals. So, slope just change in Y over change in X, pressure two minus pressure one divided by depth one two minus depth one. Plug in all those numbers and you get 12,500 pascals per meter. Now, class, what are the units on pascals? That's right, we're going to walk our way through units. So, class, again, I'm asking you to answer the question. What are the units on pascals? All right, I love it. All right, we have a correct answer already. Uh it is pressure equals force divided by contact area, so it is newtons per meter squared. So, those are the units on pascals. So, I'm going to walk my way through the units here cuz units are very important that you understand. So, pressure equals force divided by area that the force perpendicular divided by area, so that is pascals equals newtons per meters as we just talked about. So, from here, we can substitute that into our equation for pascals over meters, and we get neuter newtons, excuse me, over meters cubed. A Newton is a kilogram times meters per second cubed seconds squared. All of that divided by meters cubed. So, then we end up with kilograms over meters squared times seconds squared. So, the slope is equal to 12,000 uh 12,500 kilograms times meters squared over seconds squared, and that is equal to density, the fluid density times gravitational field strength. Again, we can solve for the density, we can substitute in that slope, and the gravitational field strength, and we end up with 1,250. All the units actually the square the second squared cancel out, and we have meters times meters squared in the denominator. We end up with 1,250 kilograms per meters cubed. Clearly, you didn't have to walk through the units to understand the solution to this multiple choice question, but please remember I am always here to help you understand what's going on, not just learn for the exam. And those units will help you out. Learning all of the units and understanding the units are going to help on the exam. So, uh the correct answer is D. Beautiful. Uh oh, by the way, uh this I guess is a large reservoir that must contain glycerin cuz this is the density roughly the density of glycerin. All right. So, question four before we move on to question four, of course, we'll see how we're doing. Ooh, the same top three. Brilliant elephant still at the front at the still at the top. Okay. Uh let's move on to the next question. We have question four looks like this. Good luck, everybody.

### [25:00](https://www.youtube.com/watch?v=Ffh1OwT2a9k&t=1500s) Segment 6 (25:00 - 30:00)

All right, ladies and gentlemen, well, choice B is correct, and again, we have most of the a lot of people answered choice B, but we have more it's uh getting harder, I guess. Okay. So, um I'm going to answer a question that was asked in the chat before we move on. Um so, in the AP exam, do we use 10 or 9. 81 for gravity? First off, never ever on the AP exam refer to anything as gravity. There's nothing that is simply gravity. There is the acceleration due to gravity, which is the same as gravitational field strength. There is the gravitational potential energy, there is the force of gravity, but please never refer to anything as gravity. For the acceleration due to gravity here on planet Earth, on the AP exam, you can simply use 10. Everywhere, multiple choice, free response questions. This is not a math test, this is a physics test. So, to make all the calculations easier, simply use 10. I encourage you, please use 10 for the gravitational field strength. All right, glasses back on. All right, here we go. How do I hide that? Doo doo, where'd it go? All right, hide that, and we go back to here. Okay, so now, we're going to figure out why the correct answer for choice for question four is choice B. Okay, here we go. A coo cube floats on Rebel. We already read the question. Forces on the cube are the upward buoyant force and the downward force of gravity. When we sum the forces in the Y direction, we have the force of buoyancy minus force of gravity, which is equal to the mass times the acceleration in the Y direction of the cube. cube is zero because it is not moving up or down. Therefore, the buoyant force and the force of gravity are equal in magnitude. Great. So, the weight of the displaced water is the buoyant force. And the weight of the cube is the force of gravity. So, the correct answer is B. That's why that's the correct answer. Let's walk through why the other answers are incorrect. So, choice A, the pressure force exerted by the water on the bottom face of the cube. Believe it or not, the force perpendicular is called the pressure force. This uh in the pressure equals the force perpendicular divided by the contact area, that is called the pressure force. I don't use it all that often, but it is called the pressure force. But, and the force buoyancy is the net force from pressure on all sides of the cube, not just the bottom. So, choice A is incorrect. For choice C, we have the equation for force buoyancy, which we've already walked through. The volume of the fluid dis is the vol V sub F is the volume of the fluid displaced by the object, not the volume of the object {slash} cube. So, choice C is incorrect. Choice D, pressure and force are two different things, so pressure and force will not have the same magnitude. So, choice D is incorrect. Yes, they could have the same magnitude if the area were 1 m squared, but it doesn't say anything about that in the problem, So, don't even ask. All right. Sorry. Uh moving on to question five. All right. Let's move Actually, sorry. Let's see how we're doing.

### [30:00](https://www.youtube.com/watch?v=Ffh1OwT2a9k&t=1800s) Segment 7 (30:00 - 35:00)

Ooh, the top three are still the same. Interesting. All right. Moving on to question five. Question five looks like this and next. All right. Okay, this is a fun one. So, choice D is the correct answer, but we have 29 people who answered choice C. This is going to be fun. Let's figure out why. Yeah. — [clears throat] — Just so you know, I I knew that was going to happen. Okay, here we go. Sorry. Here we go. Uh choice five. I'm sorry, question five. Here we go. So, choice A. Mass one equals mass two as stated in the problem. So, choice A is incorrect. Now, there will be people who choose this answer. Realize that I understand it says literally that the two objects have the same mass and choice A says that they don't. Please, do not deride or put down anyone who or yourself who chooses who chooses answer. Really, it's going to happen, okay? This These questions can be stressful and confusing. This whole thing can be confusing and we have all been there. So, please give yourself a little bit of slack. Give other people slack. I realize there are going to people answer choice A, but it is incorrect. Choice B. Yes. Buoyant force The buoyant force on object one is greater than two. However, object death depth does not affect the buoyant force. So, choice B is incorrect. Choice C. The buoyant force is not determined by the volume of the object, but rather fluid displaced by the object. So, choice C refers to the volume of the object, not the volume of the fluid displaced by the object. So, choice C is incorrect. No. It could be true that object one has a larger volume and therefore lower density. However, it does not have to be true. So, choice C is not the best explanation. Please. Choice D. Object one floats because it has a larger buoyant force, right? That's why it floats. So, we've already walked our way through the buoyant force equation equals the density of fluid displaced by the object times the volume of the gravitational field strength. Both objects float in the same fluid, so the density of the fluid is the same. The only way for object one to have a larger buoyant force is for the volume of the fluid displaced by object one to be larger. So, choice D is correct.

### [35:00](https://www.youtube.com/watch?v=Ffh1OwT2a9k&t=2100s) Segment 8 (35:00 - 40:00)

Okay. So, I am going to return to choice C because I'm guessing that there are some of you are still unconvinced, right? I I'm fully aware that there are some of you who are going to be unconvinced. So, let's walk our way through it. So, choice C. I'm going to give you an example of two objects where they have the same mass, are in the same fluid, and the object with the larger volume and therefore smaller density is the one which sinks. Here we go. Let's say we have two spheres of equal mass. One sphere is made of aluminum and steel. So, therefore, the density of aluminum is less than the density of steel. Therefore, the volume of the aluminum is going to be volume the greater than the volume of the steel. Again, because they all have the same mass and the density of aluminum is less than the density of steel. So, the aluminum sphere is placed in a tank and it sinks to the bottom. It looks like that, right? The aluminum sphere is stuck at the bottom of the tank. Great. So, we're going to take a hydraulic press and we're going to make the steel sphere into a steel ball. The steel ball is placed gently in the water. So, the volume of the fluid displaced by the steel is much greater than the volume of the steel. So, the steel ball floats. It looks like this. So, this right here is an example where we have two objects where they have the same mass, are in the same fluid, and the object with a larger volume and therefore smaller density is the one which sinks. And actually, they could both be balls, right? You could just put the aluminum ball in such that it gets filled with water and therefore sinks and you could gently put the steel ball in so it floats. So, please please realize that. Okay. — Okay. Um All right. So, we are going to move on to question six. Uh actually, before we do, of course, we are going to see how everybody's doing. Interesting. Scoreboard stayed the same. All right. Um moving on to question six. And right. And clicking next. All right, here we go. Choice C. Good, a lot of people answered choice C. Before I answer we go through why choice C is the answer, I'm going to answer some of these questions. Uh let's see. Do do do. Let's go Let's start with this one. Uh can I come to your high school and and teach you AP Physics? Uh sadly, no, but I have 40 hours of videos on YouTube where you can learn from me. So, essentially, yes, but sorry, no.

### [40:00](https://www.youtube.com/watch?v=Ffh1OwT2a9k&t=2400s) Segment 9 (40:00 - 45:00)

Um so, uh so, I do feel like I I'm not going to go through this because I do feel like this was a very good answer for that. So, I'm going to leave that be. Uh I am going to talk about this, though. Are these the types of questions that will be on the AP test? So, yes, these are the types of questions that are going to be on the AP test. I think this is gets to um yeah, which is So, yes, these are the types of questions that are going to be on the AP test. I try to mix a good amount of I try to actually come up with the same roughly the mix that you'll see on the AP test. I do tend to make my multiple-choice questions and free-response questions a little bit more difficult than I would expect you to see on the AP exam uh cuz I always feel like it's better to have things be a little bit more difficult, but there you will see some very difficult things on the AP exam, as well. So, just keeping you updated there. Um all right, so, let's come back to here. And let's go through why the answer to question six is I don't remember which one it was. Uh C, I think it was C. All right, here we go. Question six. So, the total mass both have a mass of capital M, so the total mass of both of them is two times capital M. The density of one is lowercase rho, and the density of two is two times lowercase rho. We know density equals mass divided by volume, so volume density. So, the total volume is the volume of one plus the volume of two. So, we can add both of those mass divided by density for both of those. We can substitute in the values we got. So, capital M over lowercase rho plus capital M over two times lowercase rho, right? This is for the first object, this is for I'm sorry, the first fluid. This is the for the second fluid. And in order to add the two together, we just do that. So, we end up with three halves capital M over rho. That is the total volume of both liquids. So, the total density then is going to be the total mass divided by the total volume, or two times capital M divided by the quantity two times lowercase rho. That gives you four times rho divided by three, or one with a three 1. 3 repeating, or roughly 1. 3 times rho. So, the correct answer is C. Great. Moving on to question seven. But before we do, let us see. Okay. All right, the top three have been the top three for a while. We will see. All right. Choice D, a lot of people got it correct. That's great. Uh before I go through why choice D is correct, let me just answer a couple questions. So, first off, what is the best review study material for AP Physics 1 and 2 exam? For AP Physics 1, it is Ultimate Review Packet and Ultimate Exam Slayer. It's what I sell. Honestly, I have to say it's the best cuz I made

### [45:00](https://www.youtube.com/watch?v=Ffh1OwT2a9k&t=2700s) Segment 10 (45:00 - 50:00)

it. So, yeah, absolutely. Um yeah, it's mine. Uh for AP Physics 2, unfortunately, there just isn't a whole lot out there, and I don't have any suggestions for AP Physics 2. I am really sorry. So, um I do want to go through Oh, where did it go? Um this question. What are the structure of the free-response questions? So, in the Ultimate Exam Slayer, I have like 40 minutes of videos where I go through the structure for all of the free-response questions, talk about how they're set up and how to answer the questions and all of that. So, that is something that would be very helpful for you. Just going to say that. Okay, here we go. Question seven, choice D. Here we go. Okay. Question seven. Class, can anyone tell me what the reading on the force sensors are equal to when the spheres are in the water in this setup? So, in this exact setup, what are the readings on the force sensor equal to when the spheres are in the water? General question for everyone. I'm curious if people know the answer. Ah. — [sighs] — All right, this is fun. Okay, we got a couple. So, um this is not correct. It's not the force of gravity minus the force buoyant. It is Right, gravity. Somebody put gravity in here. Um it is also I love this. Okay. So, very close. It is not the buoyant force. It's very close to the buoyant force. We're going to walk away through this. All right, beautiful. Okay. It's very close to the buoyant force, but not quite. So, here we go. The force readings are equal to the negative of the buoyant force. So, it's equal to the magnitude of the buoyant force, but it actually ends up being a negative value, so it is equal to the negative of the buoyant force. It's important that you understand this. I'm going to walk my way through this so you understand why it the force readings are going to be equal to the negative of the buoyant force. So, before the spheres are lowered into the water, the forces acting on each sphere are the force of tension, which is up. And the force of tension will change when the sphere is in the water, so we're going to call that force of tension the initial force of tension. The force of gravity is down, but the force of gravity does not change when the sphere is in the water, so it's just going to be the force of gravity. And yes, there are two spheres, but for now, we're I'm just talking about a generic sphere, right? So, we're sphere when it goes in the water in this situation. So, when we sum the forces initially in the Y direction, this is before we lower it into the water, the force of tension initial minus force of gravity equals mass times the acceleration in the Y direction. direction of the sphere isn't moving up or down, so the acceleration in the Y direction of the sphere is equal to zero. So, the force of tension initial is equal to the force of gravity. Okay. But then, we zero out the force sensor. So, as far as the force sensor is concerned, that initial force of tension is zero. So, we've zeroed out that initial force of tension. That's what zeroing out the force sensor does. So, after the spheres are lowered into the water, the forces acting on the sphere on each sphere now are we have the force of buoyancy, which is up. Again, we have the equation for force buoyancy, which equals the density of the fluid displaced times the volume by times the gravitational field strength. Again, with that F for fluid displaced by the object. So, this is the density of the fluid displaced by both spheres, which is the same because they're both in the same uh fluid, which is water. And the volume of the object for the volume for both spheres is the same, so the volume of the fluid displaced by both spheres is the same. Therefore, the force buoyancy acting on both spheres is the same. The force of tension is up. The sphere is in the water, so the force of tension has changed. So, this the I'm now going to label it the force of tension final. So, the force of gravity is down, and it is the same as before. So, now, when we sum the forces final after the spheres are in the water, we get in the Y direction, the force of tension final plus the force of buoyancy because those are both up, minus the force of gravity because that's down. Again, the acceleration in the Y direction is equal to zero because the spheres are not moving up or down. Therefore, the force of tension final equals the force of gravity minus the buoyant force. Right. So, the reading on the force sensor is equal to the change in the force of tension in the string. So, the change in string is the force of tension final minus force of tension initial. We can substitute in. We got for the force of tension final, force of gravity minus force buoyancy, and for the force of tension initial, we have the force of gravity. So, force of gravity cancels out and we get the negative of the buoyant force. So, both force sensor readings are equal to the negative of the buoyant force, and both force

### [50:00](https://www.youtube.com/watch?v=Ffh1OwT2a9k&t=3000s) Segment 11 (50:00 - 55:00)

buoyant forces are the same. So, both force So, the correct answer is D. So, realize the force sensor reading is negative because the force buoyancy decreases the tension force and with an initial tension force of zero. So, if it's decreasing the tension force, it's actually going to be end up being negative. All right. Here we go. Before we move on to question eight, we need to see if there's been any change in the top three. Here we go. Oh, brilliant elephant has fallen. Okay, here we go. Congratulations, Fearless Narwhal. Good luck, Lucky Koala. All right, here we go. Moving on to question eight. Question eight looks like this and next. — [clears throat] — All right, choice A is the correct answer and we have fewer people answer choice A. So, it'll be fun. Uh I do want to answer this question uh before we move on. Uh I'm a self-studier. I want to know if I have any advice what for the exam day. So, um yeah, somebody else actually already mentioned it. So, uh I have a 30 just over 30-minute exam cram where I try to go through as much of the exam material for AP Physics 1 as I can in 30 minutes. So, I definitely suggest that the morning of. Uh I also definitely suggest getting my ultimate exam slayer where I have two practice exams. So, the day before the exam, I would work your way all the way through or maybe 2 days before the exam, work your way all the way through that second practice exam. So, please. Yeah, I would definitely suggest doing that. Uh question eight. So, we're trying to figure out why choice A is correct and fewer people got that right. So, we'll figure out why. All right, choice A. Choice uh question eight. Fluid pressure increases with increased fluid depth, right? We know this. The absolute pressure equals the pressure at the top P0 plus density of the fluid times gravitational field strength times the depth, right? So, if the fluid were at rest, we would know pressure two is greater than pressure one. However, the fluid is not at rest. So, we have some information, right? If the fluid were at rest. So, from the continuity equation of ideal fluid flow, we know that the cross-sectional area at point one times the flow speed through point one is equal to the cross-sectional area at point two times the volume of the fluid uh the I'm sorry, the fluid speed uh through point two. So, we know cross-sectional area at two is less than cross-sectional area one.

### [55:00](https://www.youtube.com/watch?v=Ffh1OwT2a9k&t=3300s) Segment 12 (55:00 - 60:00)

Therefore, the flow speed at two is greater than flow speed at one. From Bernoulli's equation, I'm not going to read it, but you know Bernoulli's equation. So, if the heights are the same, if the height at one is equal to the height of two, then we get this. The this uh part of this uh expression cancels out and we get just this for the equation. So, notice that we already know the velocity through at point two is greater than velocity through point one. Therefore, we would know that the pressure at two is less than pressure at one from Bernoulli's equation again if the two heights were the same. However, height one does not equal height two. So, in summary, we know that pressure two is greater than pressure one or pressure two is less than pressure one, but that is going to be based on the numbers in the problem. So, the correct answer is A. Right? You don't know because you don't have numbers for this problem. Okay. I do want to I want to answer I There was one question I want to Here we go. go. I just want to go through this. First off, I've answered why it's A. But, I've seen a lot of this, how we're cooked, etc. You got 2 weeks. You are not cooked. Yes, at times it's going to feel like you do not understand what's going on. You got to take the time. You got to practice, practice. My ultimate review packet, ultimate exam slayer, I I can't stop bringing them up because there's so much in there that you can practice. And you got to take the time to practice. You are not cooked, but you got to do you got to put in the work. I'm sorry. And yes, some of these questions are going to be hard. That's my job. You're welcome. All right, here we go. Uh let's see. We are moving on to question nine. Oh, actually let's get rid of this first. Do do. All right, question nine. Let's see before we do, of course, let's see if Fearless Narwhal is still at the front. It is all you. It is yours to lose, Fearless Narwhal. You have two questions left. Good luck. Uh moving on to question nine. Question nine looks like this. You're welcome. — All right, choice C. We barely have more people answering choice C than anything else. That makes me happy, I guess. Okay. — All right, here we go. Let's see why choice C is correct. So, volumetric flow rate, as we just discussed in the last question, equals the cross-sectional area times the flow speed through that cross-sectional area.

### [1:00:00](https://www.youtube.com/watch?v=Ffh1OwT2a9k&t=3600s) Segment 13 (60:00 - 65:00)

Now, class, what else is volumetric flow rate equal to? What else volumetric flow rate is equal to? I love this sentence. I'm going to read it. Class, what else volumetric flow rate is equal to? I wrote that sentence. — All right, we have some people who know the answer to this question. Here we go. So, it is equal to ba ba volume over change in time. So, volumetric flow rate is not just equal to cross-sectional area times the flow rate. Both of those are going to be at the spigot. It's also equal to volume over the change in time. So, the volume of the fluid divided by the change in time it takes to get to that amount of volume of fluid. So, the change in time, which is what we're solving for, is equal to the volume of the cube divided by the cross-sectional area of the spigot times the the speed of the flow speed through the spigot. I apologize for the uppercase and lowercase V's, but you're going to have to handle it. Here we go. So, the radius of the spigot is 4 cm. We can convert that over to meters, multiply by 1 m over 100 cm, and we can then figure out the area of the spigot. It equals this number right here. I'm not going to read that for you. So, the volume of a cube is just the side cubed or 0. 95 cubed or 0. 857 m cubed. So, the height here for your Torricella Torricelli's equation is going to be the depth of this fluid, right? It is going to be the 12 m minus the 5 m or 7 m. It is going to be this depth right here. So, the flow speed at the spigot is going to be two times gravitational field strength of 10. We're using 10 * 7, which is going to be 11. something m per second. So, then the cross-sectional area of the spigot times the flow speed of the spigot, we multiply those two numbers together and we get that giant number in meters cubed per second. So, then the change in time, we can substitute in the numbers, so that we get meters cubed over seconds, and that works out to be seconds or roughly 14 seconds. — [snorts] — The correct answer is C. Just so you know, a common mistake is to use H = 12, and that gives you incorrect answer choice B, which is the second most chosen one. There you go. All right. Um actually, let's do this cuz we can answer this. Great question. Nope. Sorry, that's not the one. Okay, here we go. How do we get this all in 2 minutes time? So, the answer is there will be questions which will take more than 2 minutes. There will be questions that take less than 2 minutes. Uh I in this, I'm just giving you 2 minutes so you get a good feel for how long 2 minutes is. Uh but realize, yes, this one will probably take a little bit more than 2 minutes, but there are other questions that are going to take less than 2 minutes. So, but you do need to practice because speed is going to be important for this exam. So, you do need to move at a rather quick pace. So, you do need to practice. And again, in my ultimate exam slayer and ultimate review packet, I have a total of three different practice exams. Please utilize them. All right. Uh here we go. Let's see how we're doing. Fearless Narwhal? Nice. I'm impressed. Uh last question. Question 10 looks like this.

### [1:05:00](https://www.youtube.com/watch?v=Ffh1OwT2a9k&t=3900s) Segment 14 (65:00 - 70:00)

Awesome. — I like ending that way. That feels good. All right, good job, everybody. Uh I like to ask some questions before we figure out why choice A is correct. Okay. So, is the Torricelli's equation theorem worth memorizing? Uh so, I would say it's easy enough to memorize that sure. Really, the main thing is to know how to derive it because most likely you're going to have to derive it. You As an example, I tried to give a good example there, uh which is most likely on a multiple choice question they're actually going to give it to you because we're not a big fans of memorization, but it's fair enough to memorize. But really, you do need to know how to derive it. Um Yeah. Um so, I also want to uh piggyback on what Ms. Bonk said here. So, I completely agree that the biggest piece of advice for multiple choice questions and how to get faster and how to do them better is to just do as many of them as you can and learn from your mistakes. And you need to get way used to the way the College Board words things. So, I got I got I'm going to push it again. In my I'm going to do it. Darn it, I'm going to make it so you can see me. — [snorts] — Um on my Wait, let's do this. I have never done this before. Haha. So, in my um ultimate review packet ultimate exam slayer, I have In my every uh I have unit uh multiple choice quizzes for every unit. And so, these first 10 are the first 10 in those multiple choice unit quizzes, but then I have at least 15 more. And I have all the solutions to everything. So, everything we're talking about here, I actually have as solutions in there, so you could see all of that. In addition to that, I actually have more than what we talk about here just because I can't quite talk about everything. And I have the all those multiple choice questions in the practice exams. So, please utilize all that and look through all of the solutions. Don't just even if you get it right, look through my solution cuz I have all sorts of really useful things that I say in my solutions. So, please please ultimate exam slayer ultimate review packet packet. Use them. Please. All right, we're back here. Okay. Um so, let's go through this question. All right, why is it A? So, question 10. So, in question 10, choice A. So, the continuity equation of ideal fluid flow, we've already talked through it. The cross-sectional area times the flow speed is constant. So, because um it's a circular pipe, well, that's the cross-sectional area of a circle is pi r squared. So, we can solve for the flow speed, which is on the Y axis. That's equal to a constant divided by pi times r squared. We can factor out the constant divided by pi because these are just numbers, that times 1 over r squared. So, this matches the shape of a graph for y = 1 over x squared. Right? So, the correct answer is choice A. Uh choice B is Bernoulli's equation. Flow speed is in the equation, however, the radius of the pipe is not. So, there's no way to graph that, really. So, choice B is incorrect. Choice C is Torricelli's equation equation. So, the flow speed, yes, the radius of the pipe is not in there, so choice C is going to be incorrect. Uh and choice D is a rearrangement of the contact pressure equation, which we talked about before. So, um again, you've got the flow speed is in there, the radius of pipe is in there, but uh the I'm sorry, the flow speed is not in there. The radius of pipe is there in there, but choice So, choice D is incorrect. So, and we are done. Let me say a couple things before I show the results. My plan is this for this video to remain live on YouTube.

### [1:10:00](https://www.youtube.com/watch?v=Ffh1OwT2a9k&t=4200s) Segment 15 (70:00 - 73:00)

I see no reason to take it down. So, this multiple-choice quiz is going to be posted in my Ultimate Exam Slayer. Realize there are 16 more questions in my quiz for Ultimate Exam Slayer for unit 8, and as I talked about, there are at least 15 or 16 in every unit quiz and the practice exams and all that stuff. So, again, I'm going to have a 14 multiple-choice all AP Physics 1 Kahoot um quiz um again, 1 week from today at 7:00 p. m. And then, for exclusively for my Ultimate Exam Slayers, 1 fortnight from today, we're going to have 14 multiple-choice again, all AP Physics 1 Kahoot. Again, those are going to be completely different than the ones from uh the one in 1 week. So, let us see the leaderboard. Actually, let me see if there are any questions. Do do I want to answer? Yeah, we'll do it after. All right, so here we go. Um next, let's check the leaderboard. Lucky Koala in third place. Brilliant Elephant, second. And uh — [sighs and gasps] — Fearless Narwhal. Congratulations to Fearless Narwhal. Okay, so here we go. Are all the AP Physics 1 exam questions this hard? So, no, not quite, but there will be exam questions that are this hard. Uh I did my best to um model them, but I tried to make them a little bit harder, mine a little bit harder than the AP exam questions. But, you just be aware that the AP exam questions will be hard, and the timing is going to be difficult. Again, that's why uh that's why it's really important to practice and get better at this stuff. So, uh what's the difference between the Ultimate Review Packet, it's Ultimate Review Packet, and the Ultimate Review Slayer, Ultimate Exam Slayer? Um the multiple-choice questions. So, they have different multiple-choice questions. There are fewer multiple-choice questions in the Ultimate Review Packet than the Ultimate Exam Slayer, cuz the Ultimate Exam Slayer is more designed for um just like studying for the exam itself, whereas the Ultimate Review Packet is more for content. But, both are very useful, and if you want all three practice exams, that's definitely the way to go is to get both of them. — You're welcome. I it I don't know. It wasn't I thought it was interesting, but yeah, I agree it wasn't that hard. Um uh so, I don't know. I think it's time to be done. Let's go back to here. And ladies and gentle people, thank you very much for learning with me today. I enjoyed learning with you. Hopefully, I will see you all in 1 week. Flipping Physics

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*Источник: https://ekstraktznaniy.ru/video/49325*