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A2L Item 151

Meta-Information
Description: 
Identify the ball reaching the end of its track first.
Goal: 
Reason with kinematics
Level: 
High School
Level: 
Undergraduate
Keywords: 
Dynamics
Keywords: 
Mechanics
Keywords: 
Reasoning
Source: 
UMPERG-ctqpe75

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?

  1. ball on track A
  2. ball on track B
  3. they reach the end at the same time
  4. 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.