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# 15sep08 - A truck pulls a trailer with a horizontal force...

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Unformatted text preview: A truck pulls a trailer with a horizontal force. The reaction to the force that the truck exerts on the trailer is (A) The horizontal force exerted on the trailer by the ground. (B) The force exerted by the trailer on the truck. (C) The horizontal force exerted on the truck by the ground. (D) Air friction on the truck and trailer. (E) The horizontal force exerted on the ground by the trailer. grpi Latc‘ F {Mia m tlietmtli Pewter cu dIt' truck Can an object move at constant velocity when only a single force acts on it? A. Yes; the object will act back with an equal and opposite force. B. Yes; at least one force is necessary to keep an object in motion. 0. No; at least one other force is needed to cancel the action of the first force. D. None of the above. CoﬂS'fn/tt r dud/T Hiiitc'vr : C A locomotive pulls a series of wagons. Which is the correct analysis of the situation? 1. The train moves forward because the locomotive pulls forward slightly harder on the wagons than the wagons pull backward on the locomotive. 2. Because action always equals reaction, the locomotive cannot pull the wagons-the wagons pull backward just as hard as the locomotive pulls forward, so there is no motion. 3. The locomotive gets the wagons to move by giving them a tug during which the force on the wagons is momentarily greater than the force exerted by the wagons on the locomotive. 4. The locomotive's force on the wagons is as strong as the force of the wagons on the locomotive, but the frictional force, on the locomotive is forward and large while the backward frictional force on the wagons is small. 5. The locomotive can pull the wagons fonuard only if it weighs more than the wagons. force-3 m u;.;iiit-‘rna_ Nmmﬁ; jntttxm/ 6‘4“? .- F / wt Mm from wagons l‘n‘te‘s m wont/i8 . 7 :L 4' ' forum-pf %‘%J—* > "Him 1):». ma, .1 . . n. W Bird L ~ r— Ffivn mowing 3: it E' \ lYn) wg ‘ *4] 1‘ l '7‘ ’l 1‘“ Consider a car at rest. We can conclude that the downward gravitational pull of Earth on the car and the upward contact force of Earth on it are equal and opposite because 1. the two forces form an interaction pair. Thim [it it ,‘ 2. the net force on the car is zero. 3. neither of the above Answer: 2 These two forces cannot be an interaction pair because they act on the same object. Because the car is at rest, however, its momentum is constant (and zero). Because net forc equals the time rate of change in momentum, the net force on tht car must be zerr Flu r . “at the two forms must be equal ...
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