Goal: Reason with kinematics
Source: UMPERG-ctqpe76
Two identical steel balls are released from rest from the same height,
and travel along tracks as shown and labeled below.
Which reaches the end of its track first?
- Ball on track A
- Ball on track B
- They reach the end at the same time
- Not enough information
Commentary:
Answer
(2) The ball on track B accelerates down the second slope. A component
of this acceleration is in the x-direction. This means that the x
component of ball B’s velocity is never smaller than that of ball A.
Since the tracks have the same x-dimension, ball B gets there first.
A large majority of students choose answer C incorrectly thinking that
since the balls return to the same height, they have the same speed and,
therefore, arrive at the same time.