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From: "Carl E. Mungan" <mungan@USNA.EDU>Okay.
True enough for a wheel without a drive shaft. But if there is a
drive shaft attached to the wheel, I can craftily apply just the
right pressure on the gas and brake pedals to keep the wheel spinning
on ice in such a way that it always matches the rolling without
slipping condition.
You would probably protest that not's really rolling. But how aboutI'll have to think about this one.
the following then: if I have a spool (with a suitable moment of
inertia) and I pull on the thread at just the right angle, I can get
the spool to roll (starting from rest) even on a frictionless surface
(without slipping), can I not? (Sorry I couldn't resist bringing this
up ;-)
Which is to say, friction is not really required for rolling. Nor isRight. Friction opposes the direction of slippage.
it true that if I apply a force to an object to get it to move on an
arbitrarily rough surface that the friction must oppose the motion.
The friction could be zero or even in the direction of motion. I tellRight. Foot exerts down-and-back force on floor and floor exerts up-and-forward force on foot, the horizontal component of which is the friction force that opposes the foot's direction of slippage. Yet the foot, and the body it's attached to, it accelerated forwards. This one usually makes students stop and scratch their heads.
the students I'll demo this. I then walk towards them and tell them
the demo is over.