Chapter 5: Q11 CQ (page 138)
Can an object exert a force on itself? Argue for your answer?
No, it can’t. If it could, objects would spontaneously accelerate and the world’s energy problem would have been solved a long time ago.
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Twenty people participate in a tug-of-war. The two teams of ten people are so evenly matched that neither team wins. After the game, they notice that a car is stuck in the mud. They attach the tug-of-war rope to the bumper of the car, and all the people pull on the rope. The heavy car has just moved a couple of decimeters when the rope breaks. Why it did not break when the same twenty people pulled on it in a tug-of-war?
Question: A truck loaded with sand accelerates along a highway. The driving force on the truck remains constant. What happens to the acceleration of the truck if its trailer leaks sand at a constant rate through a hole in its bottom? (a) It decreases at a steady rate. (b) It increases at a steady rate. (c) It increases and then decreases. (d) It decreases and then increases. (e) It remains constant.
A car is moving forward slowly and is spending up. A student claims that “the car’s engine exerts a force on itself” or that “the car’s engine exerts a force on the car.”
(a) Argue that this idea cannot be accurate and that friction exerted by the road is the propulsive force on the car. Make your evidence and reasoning as persuasive as possible.
(b) Is it static or kinetic friction?
Suggestion: Consider light gravel. Consider a sharp print of the tire tread on an asphalt road, obtained by coating the thread with dust.
Question: A block of mass 2.20 kg is accelerated across a rough surface by a light cord passing over a small pulley as shown in Figure P5.99. The tension T in the cord is maintained at 10.0 N, and the pulley is 0.100 m above the top of the block. The coefficient of kinetic friction is 0.400. (a) Determine the acceleration of the block when x = 0.400 m. (b) Describe the general behavior of the acceleration as the block slides from a location where x is large to x= 0 . (c) Find the maximum value of the acceleration and the position x for which it occurs. (d) Find the value of x for which the acceleration is zero.
Question: Before 1960, people believed that the maximum attainable coefficient of static friction for an automobile tire on a roadway was . Around 1962, three companies independently developed racing tires with coefficients of . This problem shows that tires have improved further since then. The shortest time interval in which a piston-engine car initially at rest has covered a distance of one-quarter mile is about . (a) Assume the car’s rear wheels lift the front wheels off the pavement as shown in Figure P5.58. What minimum value of is necessary to achieve the record time? (b) Suppose the driver were able to increase his or her engine power, keeping other things equal. How would this change affect the elapsed time?
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