A2L Item 141

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

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The question for students:

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

Which goes further before stopping?

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

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Commentary for teachers:

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.

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