Teaching Physics with the Physics Suite

Edward F. Redish

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Closing the door

A student is in her dorm room, sitting on her bed doing her physics homework. The door to her room is open. All of a sudden, she hears the voice of her ex-boyfriend down the hall, talking to the girl in the room next door. She wants to shut the door quickly, so she throws a superball (which she keeps next to her bed for this purpose) against the door. The ball follows the path shown in the top-view diagram below. It hits the door squarely and bounces straight back.

  1. If the ball has a mass m, hits the door with a speed v, and bounces back with a speed almost equal to v, what is the change in the ball's momentum?
  2. If the ball was in contact with the door for a time Δt, what was the average force that the door exerted on the ball?
  3. Would she have been better off with a clay ball of the same mass stuck to the door? Explain your reasoning.
  4. Does the ball's effectiveness in closing the door depend on where on the door the ball hits? If it does, where should it hit to be most effective? Explain your reasoning.

Note to instructor: This is a final exam-style version of a Workshop Physics problem. It leads the student a bit into thinking about momentum's role in determining force -- good for an exam where there is time pressure, but the WP version is better for homework. This also crosses topic boundaries, bringing in the idea of torque in the last part. The students should be able to answer this from their personal experience even if torque has not yet been discussed.

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Page last modified December 4, 2004: P&E16