CH9-QP Answers.doc - CHAPTER 9 Static Equilibrium Elasticity and Fracture Answers to Questions 1 If the object has a net force on it of zero then its

CH9-QP Answers.doc - CHAPTER 9 Static Equilibrium...

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CHAPTER 9: Static Equilibrium; Elasticity and Fracture Answers to Questions 1. If the object has a net force on it of zero, then its center of mass does not accelerate. But since it is not in equilibrium, it must have a net torque, and therefore have an angular acceleration. Some examples are: a) A compact disk in a player as it comes up to speed, after just being put in the player. b) A hard drive on a computer when the computer is first turned on. c) A window fan immediately after the power to it has been shut off. 8. The mass of the meter stick is equal to that of the rock. For purposes of calculating torques, the meter stick can be treated as if all of its mass were at the 50 cm mark. Thus the CM of the meter stick is the same distance from the pivot point as the rock, and so their masses must be the same in order to exert the same torque. 13. When you rise on your tiptoes, your CM shifts forward. Since you are already standing with your nose and abdomen against the door, your CM cannot shift forward. Thus gravity exerts a torque on you and you are unable to stay on your tiptoes – you will return to being flat- footed on the floor. . Solutions to Problems 3. Because the mass m is stationary, the tension in the rope pulling up on the sling must be mg , and so the force of the sling on the leg must be mg , upward. Calculate torques about the hip joint, with counterclockwise torque taken as positive. See the free-body diagram for the leg. Note that the forces on the leg exerted by the hip joint are not drawn, because they do not exert a torque about the hip joint.
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