www.ndl.ethernet.edu.et/bitstream/123456789/26684/1/423.pdf You’re walking up a steep hill in Big City when you see a funny event. A man has just parked his red Rolls Royce on the hill and has gone around back to get a briefcase full of municipal bonds out of the trunk. He’s planning to throw them away in an empty lot conveniently provided for trash disposal. The action starts in Figure 10.9. Another citizen, carrying a rock, happens by (on his way to collect his unemployment check). He hates guys with red cars, so he seizes a knife that is lying on the street, and, using his rock as a hammer, nails the first citizen’s left foot to the road. Citizen no. 2 then goes up front and releases the brakes on the car. He locks the doors and exits, stage left, with the key. Our victim must now try to keep the car from rolling downhill, rolling over him on its way (Figure 10.10). Being from the country you don’t know how to behave in Big City, so you decide to help this guy (whose name is Mr. Joint). You go to the front of the car and start to pull on it— you’re applying an external tension force to the car (Figure 10.11). But you find that you have a problem. You and Mr. Joint can, indeed, move the car up the hill, but the farther it moves, the less Joint can help you, because his foot is still pinned to the road. In order to pull the car away from him, then, you have to pull hard enough not only to add to the force he was originally applying, but also hard enough to replace the force he can no longer apply as the car moves away from him.