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Hot goIITian

Joined:	27 Oct 2007
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27 Dec 2007 18:45:22 IST

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Rotational Mec doubt

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Hi all,

Just while teaching the topic our sir told us that during rolling the lowest point of the wheel which is in contact with the ground is at rest. Please explain me this point. And don't give the vector analysis of Resnick and Halliday. Also specify the frame from which you are saying!.

Physical and analytical proof will be better!!!!!

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Blazing goIITian

Joined: 1 Apr 2007

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27 Dec 2007 18:52:18 IST

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it is not at rest but its displacement is zero...yaar!!! yeh bada ganda topic hai..bacche say too much abt this..but my funda is please stay away from these ...as tend to erase ur other concepts..

Mayank Sharma

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Joined: 20 Dec 2007

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27 Dec 2007 21:40:16 IST

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at the point of contact in the case of pure rolling...the point of contact is the instantaneous axis of rotation.

v=wr for pure rolling

so the net velocity is zero for point in contact from ground frame.

imagine that you are sitting on the point of contact....it will appear as if the ground is at rest....

imagine a ball rolling purely...i.e. with no slipping....it will appear that the sphere is in pure rotation about the horizontal axis passing through the point of contact.......

did it help??? if it did please reward me with a rate!!!!

Neeraj Agarwal

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27 Dec 2007 21:57:48 IST

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"there is no slipping b/w the surfaces"

this in simple language means that there is no rubbing....
but technically speaking.....it means that the displacement of the surfaces in contact is same......i.e. their velocity is same....

since the ground is at rest...so the the lowest point is at rest.....

Rajat Khanduja

Scorching goIITian

Joined: 14 Dec 2007

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27 Dec 2007 22:01:00 IST

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well done Mayank.

But i think i have a better explanation. And sorry, I can only explain it through vector method.

Check this out.

For pure rolling, the velocity of the lower most point (and the point in contact on the surface) is defined as the difference of

rw - v ( both vectors). So in case of rolling on ground, the speed of lowest most point is 0 as the surface it is in contact with is not moving.

You can also analyse it this way.

The lower most point ( under best conditions i.e. neglecting rolling friction and compression of surfaces) is at rest momentarily (for dt time).

Hope it clears your doubt. Nudge me if not.

PS : RATE ME if you find it helpful

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