Reason: It takes a process for the speed to drop to 0. When the instantaneous speed drops to zero, the braking force will be infinite, which is not in line with the laws of nature.

2. Beat the hanging quilt with a stick to remove some dust from the quilt.

Reason: The stick hit the quilt, giving it an impulse (great acceleration in a short time), and the quilt moved quickly. But because the dust is attached to the quilt, the quilt can't give the dust such a big acceleration, so some dust is separated from the quilt.

3. Several pieces fall together. Tap the bottom one quickly with a ruler, and the pieces on it almost fall together neatly.

Reason: The reason in 2 is similar.

4. After the power failure, the blades of the fan will continue to rotate for some time.

Reason: The reason in 1 is similar. It is necessary to reduce the rotational speed of the blade to zero by friction.