Add exercises from Problem Sets 9 and 10 (Chapter 5)

Exercises from Chapter 5 (Potential and Kinetic Energy topics).

Some exercises are incomplete:
9.10: Answer does not match solution in text.
9.12: Computation of solution does not complete.
10.5: Answer does not match solution in text.

@hazelnusse Should I hold off until those are complete or does this look okay to merge? I probably won't be looking at them anytime soon.