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Chapter 7

# Chapter 7 - 7.1 Visualize Solve(a The weight lifter is...

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7.1. Visualize: Solve: (a) The weight lifter is holding the barbell in dynamic equilibrium as he stands up, so the net force on the barbell and on the weight lifter must be zero. The barbells have an upward contact force from the weight lifter and the gravitational force downward. The weight lifter has a downward contact force from the barbells and an upward one from the surface. Gravity also acts on the weight lifter. (b) The system is the weight lifter and barbell, as indicated in the figure. (c)

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7.2. Visualize: Solve: (a) Both the bowling ball and the soccer ball have a normal force from the surface and gravitational force on them. The interaction forces between the two are equal and opposite. (b) The system consists of the soccer ball and bowling ball, as indicated in the figure. (c) Assess: Even though the soccer ball bounces back more than the bowling ball, the forces that each exerts on the other are part of an action/reaction pair, and therefore have equal magnitudes. Each ball’s acceleration due to the forces on it is determined by Newton’s second law, net / , a F m ! which depends on the mass. Since the masses of the balls are different, their accelerations are different.
7.3. Visualize: Solve: (a) Both the mountain climber and bag of supplies have a normal force from the surface on them, as well as a gravitational force vertically downward. The rope has gravity acting on it, along with pulls on each end from the mountain climber and supply bag. Both the mountain climber and supply bag also experience a frictional force with the surface of the mountain. In the case of the motionless mountain climber it is static friction, but the sliding supply bag experiences kinetic friction. (b) The system consists of the mountain climber, rope, and bag of supplies, as indicated in the figure. (c) Assess: Since the motion is along the surface, it is convenient to choose the x -coordinate axis along the surface. The free-body diagram of the rope shows pulls that are slightly off the x -axis since the rope is not massless.

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7.4. Visualize: Solve: (a) The car and rabbit both experience a normal force and friction from the floor and a gravitational force from the Earth. The push each exerts on the other is a Newton’s third law force pair. (b) The system consists of the car and stuffed rabbit, as indicated in the figure. (c)
7.5. Visualize: Please refer to Figure EX7.5. Solve: (a) Gravity acts on both blocks, and where Block A is in contact with the floor there is a normal force and friction. The string tension is the same on both blocks since the rope and pulley are massless, and the pulley is frictionless. There are two third law pairs of forces at the surface where the two blocks meet. Block B pushes against Block A with a normal force, while Block A has a reaction force that pushes back against Block B. There is also friction between the two blocks at the surface.

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