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jason.farnon
jason.farnon is offline
#11
Mar10-12, 10:51 PM
P: 12
Hi Tiny-tim, Thanks I had not even considered the difference between rolling and sliding. But I am still not clear.

Quote Quote by tiny-tim View Post

this really is only to do with the difference between sliding and rolling

if we ignore air resistance etc, the car will keep going in a circle (even on an unbanked surface) without any power from the engine
You mean no additional power, after whatever gives the car an initial velocity? I can visualize that, e.g., a satellite rotating around a planet, where gravity replaces the friction of our example. But for gravity there is a law that says it always acts radially between two bodies in the direction of the segment between their centers. With friction it only acts in the direction opposite motion. This is my question, what motion is it opposite to.

Let me try this way. Tell me where I mess up. 1. When a car is driving in a circle with uniform speed, friction supplies the centripetal acceleration. 2. Friction is a force between two surfaces in contact that acts in the direction opposite to the direction of motion between the two surfaces that would exist in the absence of friction. 3. Therefore in the car example friction opposes motion in the outward radial/centrifugal direction. 4. Therefore there is a centrifugal force.