Free response · Unit 4 Beta

Latching pucks: where does the energy go?

Qualitative/Quantitative Translation · 8 points

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The question

Name: ______________________   Date: ____________   Latching pucks: where does the energy go? (8 points)

Diagram

Latching pucks

Puck A, of mass m, slides at speed v₀ toward puck B, of mass M, which is at rest on level, frictionless ice. The pucks latch together when they collide. A student repeats the experiment with pucks B of different masses M, keeping m and v₀ the same.

Puck A slides toward puck B, which is at rest, on level iceA, mass mv₀B, mass M, at restVelcro on the facing edges: when A hits B they latch and slide on together.

  1. (a) As M is made larger, does the fraction of A's initial kinetic energy that remains as kinetic energy after the collision increase, decrease or stay the same? Justify your answer without manipulating equations. [3 points]

  2. (b) Starting with conservation of momentum, derive an expression for the fraction K_f/K_i of A's initial kinetic energy that remains after the collision, in terms of m and M. [3 points]

  3. (c) Explain how your expression in part (b) supports your claim in part (a). Then use it to determine the fraction of the kinetic energy lost when M = m. [2 points]