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- Thread starter jontyjashan
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Doc Al

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Where did you see this statement?If we set a ball rolling on a loop-the-loop track,why do we say thatwe need infinite friction coefficientfor the ball to complete the loop.

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Seems nonsensical. Wouldn't an infinite coefficient of friction say that you can't move an object at all?

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rcgldr

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It just would mean there's no slippage while the ball is rolling. Infinite friction isn't required. You just need enough friction combined with enough speed and the related centripetal force to keep the ball rolling and not sliding, even when at the top of the loop.Wouldn't an infinite coefficient of friction say that you can't move an object at all?

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That said, an infinite coefficient of static friction is not *required*- all that's needed for the ball (or any object, sliding or not) to complete a vertical loop is that the normal force does not vanish.

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Doc Al

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Why do you think this?But if we look at the friction required on inclined plane for rolling it is some function of tan x .

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Doc Al

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Please show us!Physics says this and it can be proved easily using concept of pure rolling and solving some eqns

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Just try it once yourself and btw you can find it in any good standard physics book.

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Doc Al

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Nonsense. If it's so simple, just show us. (I suspect you are mixing up a few concepts or situations.)Just try it once yourself and btw you can find it in any good standard physics book.

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Then mgsinx-f=ma(linear)

Also equating torque we get

fR=I@(angular acceleration)

and a=R@ for pure rolling

Solving this v get

f=Ia/R^2

And a = gsinx/1 + I/MR^2

So finally we have minimum friction coefficient for pure rolling is tanx/1 + MR^2/I

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Doc Al

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OK, I see what you're saying now. That's for rolling down an incline. Note that there is no acceleration normal to the surface, so only the normal component of gravity is available to provide the normal force. So if you have too steep an incline there will not be enough friction to produce rolling without slipping. (Another way of saying that is to claim an 'infinite' coefficient is required, which is clearly unphysical.)

Then mgsinx-f=ma(linear)

Also equating torque we get

fR=I@(angular acceleration)

and a=R@ for pure rolling

Solving this v get

f=Ia/R^2

And a = gsinx/1 + I/MR^2

So finally we have minimum friction coefficient for pure rolling is tanx/1 + MR^2/I

My apologies for not seeing what you meant right away!

Things are different in the loop, since there's a minimum speed to maintain contact and there's acceleration normal to the surface. The normal force is not equal to the normal component of the weight.

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Still my query is not solved

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Doc Al

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Your statement about needing an infinite coefficient of friction was based on rolling without slipping down an incline (with infinite slope). The loop situation is different as there is acceleration normal to the surface.Still my query is not solved

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A few comments and clarifications from a self-declared relative expert

(My PhD was about friction, I have written a text book about mechanics):

1) When proposing or modeling problems where there could conceivably be slip, people often say "assume infinite friction". This doesn't mean that the problem solution depends on infinite friction, it only means you are supposed to assume there is no slip. For most such problems, like this roller coaster one, there is some finite value of friction that is big enough to make there be no slipping, in this case just rolling. You just "assume infinite friction" so as not to worry about it. They should better say, probably, assume no sliding.

2) For the real experts: Actually

- #17

cjl

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Then mgsinx-f=ma(linear)

Also equating torque we get

fR=I@(angular acceleration)

and a=R@ for pure rolling

Solving this v get

f=Ia/R^2

And a = gsinx/1 + I/MR^2

So finally we have minimum friction coefficient for pure rolling is tanx/1 + MR^2/I

You are assuming that all normal force comes from the normal component of gravity (which would be true, for a ball rolling down a constant incline). This also explains your result - if all the normal force comes from the normal component of gravity, the normal force will be zero for a vertical slope, thus the coefficient of friction would have to be infinite (and it would have to be negative during the top part of the loop, where the angle is greater than 90 degrees by the same logic). This is clearly nonsensical, since we know that a ball can traverse a loop without slipping, and an infinite friction coefficient does not exist.

In the case of a loop however, some of the normal force comes from the fact that the ball is experiencing acceleration normal to the surface due to the curvature of the path. Thus, your analysis is not relevant to the situation (since a ball going around a loop is an entirely different scenario from a ball rolling down a constant incline).

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