A2L Item 151
- Description: Identify the ball reaching the end of its track first.
- Goal: Reason with kinematics
- Source: UMPERG-ctqpe75
- Keywords: Dynamics, Mechanics, Reasoning
The question for students:
Two identical steel balls are released from rest from the same height, and travel along tracks as shown and labeled below.
Which ball 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 for teachers:
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.