This is the 2nd set of homework problems. These all come from Bloomfield's Student Supplement .

2.9. Three children are at a playground, playing with a seesaw. Two of the children are 3 years old and each has a mass of 15 kg; the third child is 7 years old and has a mass of 30 kg. The seesaw is made of a high-tech low-mass plastic, so that its mass can be ignored throughout this problem. The seesaw's pivot is in the middle of the board. The two 15 kg children sit together at the east end of the seesaw and the 30 kg child sits at the west end.

a. The seesaw balances, meaning that it experiences no net torques when the children and the seesaw are not touching the ground. Describe the nature of any possible motion of the seesaw during the periods when the children and the seesaw are not touching the ground.

b. What happens to the seesaw if the 30 kg child at the west end pushes downward on the ground?

c. One of the 15 kg children loses interest (as 3 year olds often do) and jumps off the board while the seesaw is exactly horizontal. What happens to the seesaw and the other 2 children?

d. The two remaining children decide to continue playing on the seesaw. If the 15 kg child stays at the east end of the seesaw, exactly where should the 30 kg child sit in order for the seesaw to balance?

e. The other 15 kg child returns and sits on the middle of the board, right above the pivot. The child has essentially no effect on the seesaw. Why not?

3.1. A spring bathroom scale is designed to report the amount of upward force it's exerting on the objects touching its surface.

a. When you first step on the scale, you usually have some downward velocity because you ÒlandÓ on the scale. As it slows your motion to bring you to rest, does the scale report your correct weight, or does it report more than your weight or less than your weight? b. If you stand motionless on one foot, rather than two feet, what fraction of your weight does the scale report?

c. If you jump upward, what does the scale report as you push yourself upward?

d. You stand motionless on two identical bathroom scales, one foot on the left scale and one foot on the right scale. What can you say about the weights that the two scales report?

e. You stand motionless on two identical bathroom scales stacked on top of each other. Each scale weighs 10 N. What weights do the two scales report?

3.6. You're taking a step aerobics class. In front of you is a small platform that you step onto and off of during the course of the exercises. Most of the time, one foot remains stationary on the platform and you use it to lift your body up and down.

a. As you step up onto the platform, your leg does work on your body. What characteristics of you and the platform determine how much work your leg must do?

b. How much work is your leg doing on your body as it lowers you gently back down to the ground?

c. If you let yourself drop back down to the ground, rather than lowering yourself gently, you could injure the leg you land on. Why does wearing padded shoes reduce your risk of injury?

d. The platforms are all identical and are made of a sturdy plastic that acts like a stiff spring. When you step up onto your platform it distorts downward by about 4 mm. You're curious about the weight of the person to your right, so you watch the platform as that person steps onto it. It distorts downward by 8 mm. How much does that person weigh?