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A2L Item 141

Meta-Information
Description: 
Identify the block that travels further along a rough surface.
Goal: 
Reasoning with kinetic energy and work
Level: 
Undergraduate
Keywords: 
Displacement
Keywords: 
Kinetic Energy
Keywords: 
Mechanics
Keywords: 
Work
Source: 
UMPERG-ctqpe58

Two blocks, M2 > M1, having the same kinetic energy move from a frictionless surface onto a surface having friction coefficient μk.

Which goes further before stopping?

  1. M1
  2. M1
  3. Both go the same distance
  4. Cannot be determined
Commentary

Answer

(1) For a given amount of kinetic energy M1 will have a larger velocity. The penetration distance depends upon the square of the velocity.

Many students will notice that the relationship equating the kinetic energy to work by friction has mass as a factor on both sides and will answer #3. They are assuming that the masses have the same velocity, not the same kinetic energy.