Homework due Oct. 27Name:Section:
18. The astronaut and the cream pie*
A 77 kg astronaut, freely floating at 6 m/s is hit by a large 36 kg lemon cream pie moving oppositely at 9 m/s. How much thermal energy is generated by the collision?




19. Sticky carts
Two identical carts labeled A and B are initially resting on an air track. The coordinate system for describing the system is shown. The cart on the right, cart B is given a push to the left and is released. The clock is then started. At t = 0, cart B moves in the direction shown with a speed v0. They hit and stick to each other. The graphs below describe some of the variables associated with the motion as a function of time. For the experiment described and for each item in the list below, identify which graph is a possible display of that variable as a function of time assuming a proper scale. "The system" refers to carts A and B together.
  1. the momentum of cart B
  2. the force on cart A
  3. the total momentum of the system
  4. the kinetic energy of cart B
  5. the total kinetic energy of the system
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20. 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: The students should be able to answer the last part from their personal experience even if torque has not yet been discussed.