118 The Laws of MotionQ5.7The molecules of the floor resist the ball on impact and push the ball back, upward. The actual forceacting is due to the forces between molecules that allow the floor to keep its integrity and to preventthe ball from passing through. Notice that for a ball passing through a window, the molecular forcesweren’t strong enough.Q5.8While a football is in flight, the force of gravity and air resistance act on it. When a football is in theprocess of being kicked, the foot pushes forward on the ball and the ball pushes backward on thefoot. At this time and while the ball is in flight, the Earth pulls down on the ball (gravity) and the ballpulls up on the Earth. The moving ball pushes forward on the air and the air backward on the ball.Q5.9It is impossible to string a horizontal cable without its sagging a bit. Since the cable has a mass,gravity pulls it downward. A vertical component of the tension must balance the weight for thecable to be in equilibrium. If the cable were completely horizontal, then there would be no vertical
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