Exam 2 Notes

You can pick up your graded Exam 2 from the box outside my office Lab 2 3255. They are (or were when I set them out) in alphabetical order. Some notes:

  • Questions 1 and 2 are similar: mechanical energy is conserved in each scenario (why?), they each start with the same kinetic energy, and they each fall the same distance h.
  • Question 4: each puck feels the same force over the same distance, so the same amount of work is done on each puck. So, each puck’s mechanical energy is increased by the exact same amount.
  • Question 8 and 10: what is conserved? Hint: there is no principle of conservation of rotational kinetic energy.
  • Question 11: I was the most startled by the performance on this question. Momentum is a vector quantity. In this collision, momentum is conserved (it is a collision after all), so momentum is conserved in each direction. This is not an elastic collision, however, so there’s no reason to think that the kinetic energy before the collision is the same as the kinetic energy after the collision. Hints: before the collision, the total momentum is up and to the right. So after the collision, the total momentum still has to be up and to the right. After the collision, Arlo has momentum to the left. What does that tell you about Rebecca?
  • Question 12: Try using energy conservation for the whole problem. For part c), what does the sign of the work need to be?
  • Question 13: Use Newton’s Second Law for Translations for the hanging mass. Use Newton’s Second Law for Rotations for the pulley. The tension in the rope does not equal the weight of the hanging mass (if it did, what would the acceleration be?). Check that your final answer makes sense: if there no rope, what would the acceleration of the block be? Since there is rope pulling up on the block, is its acceleration larger or smaller than it would be without rope pulling up? We did this problem in lecture, in lab, and for homework. Look at your notes.
  • Question 14: Another conservation of mechanical energy situation (why is mechanical energy conserved?), but this time with both translational and rotational motion.