It is often said that “friction always opposes motion.” Give at least one example in which (a) static friction ?causes? motion, and (b) kinetic friction ?causes? motion.
Solution 18DQ Example of static friction causing motion: Suppose there is a ball on the table and I am rolling the ball by putting the hand on the ball, the ball will start rolling. This is happening because of the static friction between the hand and ball and the static friction between the ball and table. For rolling we have to apply torque on the ball. Now I am not applying any force directly on the ball. I am just moving my hand on top of the ball and if there were no friction between the surfaces, no force could have been applied and the ball would have been remained stationary. But because of the static friction between the torque is applied to the ball and it’s rolling. This is an example that static friction causing motion. Example of dynamic friction causing motion. Suppose there is a boat on the river. Now if the water of the river is flowing, the boat will automatically start moving in the direction of the motion of the water. When water is passing by the surface of the boat, it is actually applying dynamic friction on the boat. This causing the motion of the boat. Without the dynamic friction, no force was applied and the boat would have been remained stationary. Hence this is an example of dynamic friction causing motion.
