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Unformatted text preview: Physics 101 Classical Physics Spring 2010 Homework 3 Solutions DUE IN CLASS ON FEBRUARY 4 1. A golfball is driven by a club and travels toward a distant hole. Ne- glecting air resistance, which of the following is true after the ball is hit ( while the golfball is in flight ): (a) There are no forces on the ball (b) There is only a downward force of gravity (c) There is only a horizontal force which makes it travel toward the fence (d) There is both a downward force from gravity and a horizontal force which makes it travel toward the fence (e) Its speed is a constant the entire time 2. A skydiver jumps out of an airplane, and after a short time she travels at a constant velocity (the terminal velocity ) and continues that way until opening her parachute. During this time (from the point she reaches terminal ve- locity until the chute is opened): (a) There is no force exerted by gravity, (b) There must be a force upward balancing the force of gravity , (c) There is a net force downward which causes her to fall at constant velocity, (d) There are no forces acting on her, (e) There is a net force upward which slows her down until she stops 3. In a tug of war, two men pull on a rope with 100-lb forces, in opposite directions. The tension in the rope is: (a) 100 lb , (b) 200 lb, (c) zero, (d) 50 lb, (e) 141 lb Consider the free body diagram of one of the men. There is the 100-lb force acting in say the- x direction and tension acting in the + x direction. There is no acceleration, therefore the magnitude of the tension is the same as the force applied, so the tension is 100-lb. Use the following description to answer the next 2 questions : You hold a 5 kg bowling ball while floating in the vacuum of deep space and then push it away with a force of 10 N. 4. After letting go: (a) The ball keeps going forever at a constant velocity , (b) The ball travels a long time and then eventually slows down, (c) The ball accel- erates continually at 2 m/s 2 , (d) Once you let go, there is no force on it so it cannot move, (e) The velocity of the ball must be 50 m/s There are no forces, so the velocity is constant after letting go. 5. During the time you are pushing the ball: (a) No force is exerted on you, (b) The ball exerts a force on you proportional to its mass, (c) The ball exerts a 10 N force on you , (d) You move in the same direction as the ball, (e) neither you nor the ball move Newtons 3rd law implies that since you applied a force of 10 N to the ball, the ball must exert a force of 10 N on you in the opposite direction. 1 6. As shown in the diagram, two blocks are attached by a string and are accelerating downward with an acceleration of g/ 3 . The block on top is attached by another string to some other unknown object, as shown in the diagram....
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