We didn't have time in class to answer all of period Four's questions about the homework. Here are some general comments and hints about the questions we didn't get to.
14. Use the pivot point as your point of reference for determining torques. Remember that the Torque due to gravity will act at the center of mass or 0.5length.
19. Although I suggested at one point in class that this problem might be easier if you use conservation of energy, I think it's actually easier if you use the acceleration value you determined in Q17and 18. The relevant distance is half the original separation distance; that is where they'll meet. v2=vo2 + 2ax. (#20 probably still is easier to solve with conservation of energy, but you could do it the same way you do 19.)
22. Since I is the sum of the contributions by all the parts, you can add the I for a rod and the I for a mass (MR^2) to get the total I.
23. Xcm = is similar but different to I. This question is a review question of an equation that we used back in our momentum unit.
24. The question is asking us to solve for a fraction: Krot/K total. K total = 1/2mv^2 + 1/2 Iw^2. Since it's a true roll wr=v. Convert w to v and then substitute into the fraction. v should cancel out. The other values are givens.
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