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Name: Chaochun
Status: student
Age: 20s
Location: N/A
Country: N/A
Date: 2001-2002

Hi: I am a little puzzled of the role of friction when you are walking or a car is running, though there are many messages about friction in this web site. Normally we think the wheel of car or our feet pushes backward the floor, then the floor pushes forward a force of static friction, and this serves as the motive force for car. But where is the impediment comes from? For a car, there is air resistance, for a human, air resistance is negligible. But a man can walk in constant speed, which means the total force is zero, and there is a backward force. Is it sliding friction?

I think you understand the problem and the solution quite well. Just remember that when a car or a human moves at constant speed, the total force acting on it or him is zero. So if a man is walking on a level surface with no air resistance or other force acting on him besides gravity and the floor, the floor must be pushing straight up with a force equal to his weight. Similarly for a car which, on a level road with no wind, can coast for a long ways with the car in neutral and the engine turned off. The car will eventually stop, of course. At low speeds, the main force stopping it is the force needed to deform the tires as they go around -- remember the tire is somewhat flat on the bottom and is continually being deformed as it goes around. If it were perfectly elastic, it would take no energy from the car, but tires are not perfectly elastic and energy is needed to deform and straighten them.

Best, Dick Plano

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