Question 9
For a block on a table the limiting friction is 18 N and the dynamic friction is 15 N. The block is pulled with a force of 22 N and is sliding. What is the frictional force acting on it now?
The right answer is the one marked above.
15 N. The block is moving, so the friction acting is the dynamic value — and dynamic friction does not change when the pull changes.
18 N would be right only at the instant motion began. 22 N assumes friction still balances the pull, but it cannot: that is exactly why the block is accelerating.