SM_PDF_chapter5 - More Applications of Newton's Laws...

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113 More Applications of Newton’s Laws CHAPTER OUTLINE 5.1 Forces of Friction 5.2 Newton’s Second Law Applied to a Particle in Uniform Circular Motion 5.3 Nonuniform Circular Motion 5.4 Motion in the Presence of Velocity-Dependent Resistive Forces 5.5 The Fundamental Forces of Nature 5.6 Context Connection Drag Coefficients of Automobiles ANSWERS TO QUESTIONS Q5.1 (a) m m r r r a R g = + (b) ma T mg = (c) ma f R = r r r r r r r f r FIG. Q5.1 Q5.2 (a) The friction of the road pushing on the tires of a car causes an automobile to move. (b) The push of the air on the propeller moves the airplane. (c) The push of the water on the oars causes the rowboat to move. Q5.3 As a man takes a step, the action is the force his foot exerts on the Earth; the reaction is the force of the Earth on his foot. In the second case, the action is the force exerted on the girl’s back by the snowball; the reaction is the force exerted on the snowball by the girl’s back. The third action is the force of the glove on the ball; the reaction is the force of the ball on the glove. The fourth action is the force exerted on the window by the air molecules; the reaction is the force on the air molecules exerted by the window. We could in each case interchange the terms ‘action’ and ‘reaction.’ Q5.4 The tension in the rope must be 9 200 N. Since the rope is moving at a constant speed, then the resultant force on it must be zero. The 49ers are pulling with a force of 9 200 N. If the 49ers were winning with the rope steadily moving in their direction or if the contest was even, then the tension would still be 9 200 N. In all of these cases, the acceleration is zero, and so must be the resultant force on the rope. To win the tug-of-war, a team must exert a larger force on the ground than their opponents do. Q5.5 If you slam on the brakes, your tires will skid on the road. The force of kinetic friction between the tires and the road is less than the maximum static friction force. Anti-lock brakes work by “pumping” the brakes (much more rapidly that you can) to minimize skidding of the tires on the road.
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114 More Applications of Newton’s Laws Q5.6 With friction, it takes longer to come down than to go up. On the way up, the frictional force and the component of the weight down the plane are in the same direction, giving a large acceleration. On the way down, the forces are in opposite directions, giving a relatively smaller acceleration. If the incline is frictionless, it takes the same amount of time to go up as it does to come down. Q5.7 As you pull away from a stoplight, friction is the force that accelerates forward a box of tissues on the level floor of the car. At the same time, friction of the ground on the tires of the car accelerates the car forward. Drop a stick into a running stream and fluid friction sets the stick into horizontal motion.
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