Good Answers to Test Short Answer Problems

From J3 Physics

Question: Why can we not set the “final velocity” of a dropped object in free-fall equal to zero when it hits the ground even though it obviously does come to a stop? (Sorry . . . “Because we get the wrong answer” is not a sufficient explanation!)

In a dropped object problem (where the acceleration remains constant), the v₀ is 0, so if the v_f is 0, it would mean the velocity never changed from 0, and the object didn’t move. Also, kinematics problems must be done in equal acceleration segments, meaning when the acceleration changes, we must treat this as a new segment. The acceleration of the object changes when the object hits the ground and comes to rest, so the part we are measuring is the segment from the release of the object and while the acceleration of velocity continues steadily, which ends just before the object hits the ground.

An excellent answer. CCoe