Teaching Physics with the Physics Suite

Edward F. Redish

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Al and George pushing the truck

George left the lights in his truck on while in a truck stop in Kansas and his battery went dead. Fortunately, his friend Al was there. Unfortunately, Al was driving his Geo Metro. Fortunately, the road was very flat. George was able to convince Al to give his truck a long slow push to get it up to 20 miles/hour. At this speed, George can let in the truck's clutch and the truck's engine should start up.

  1. Al begins to push the truck. It takes him 5 minutes to get the truck up to a speed of 20 miles/hour. During the time that Al's Geo is pushing the truck, draw separate free body diagrams for the Geo and for the truck. Order all the horizontal forces by magnitude from largest to smallest. If any are equal, state that explicitly. Explain your reasoning.

  2. If the truck was accelerating uniformly over the 5 minutes, how far did Al have to push the truck before George could let in the clutch?

  3. Suppose the mass of the truck is 4000 kg, the mass of the car is 800 kg, and the coefficient of friction between the vehicles and the road is 0.1. At one instant when they were trying to get the truck moving, the car was pushing the truck and exerting a force of 1000 N, but neither vehicle moved. What was the frictional force between the truck and the road? Explain your reasoning.

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Page last modified October 8, 2002: D13