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Confused Friction
Name: Chaochun
Status: student
Age: 20s
Location: N/A
Country: N/A
Date: 2001-2002
Question:
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?
Replies:
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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