The Direction of the Force of Friction Explained

The direction of the force of friction opposes sliding motion. When this disc is sliding to the right, it should be pretty clear that the force of kinetic friction from the table on the disc is to the left to oppose the sliding motion of the disc. And the force of friction decreases the speed of the disc. [snorts] But it gets a bit trickier when the disc is on top of a moving block and the disc is sliding relative to the block. — Flip [singing] physics. In this case, both the disc and the block are moving to the right relative to the stationary table. But now the force of kinetic friction from the block on the disc is to the right. And the force of friction now increases the speed of the disc. At first, it may look like this force of friction is not opposing the sliding motion. However, when we motiontrack the video to make the block stationary, it becomes clear that the disc is sliding to the left relative to the block. In other words, the direction of the force of friction opposes the sliding motion between the two surfaces. In this case, the disc is sliding to the left relative to the block. So, the force of kinetic friction from the block on the disc is to the right to oppose that sliding motion. And in this case, the force of friction increases the speed of the disc.