IIT JEE , AIEEE Forum - Physics , Mechanics

priyesh

but from 3 & 6 we get v1 equal to zero this implies that centre of disk is not moving how is that possible!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Plz explain

bvsatyaram

The disk has two horizontal forces acting on it.
1) Tension towards right
2) Friction towards right.

In this case, both turn too balance. (try checking ur self.) So, net force acting on the disc is zero... which means, the disc velocity remains constant...
Here initial velocity is zero... hence the disc's centre of mass continues to stay at rest.

priyesh

still not understood

see if the disk centre of mass is at rest this means that the disk is only rotating but not translating, but the disk has to translate as it is in constraint due to the limited amt of thread.

Plz help satyaram sir

bvsatyaram

Yeah..
Here the string is not tied to the top most point of the disc... it is wound around the disc. So, as the disc rotates, the string gets unwound and the length of the string increases... here length of the string is not constant..

priyesh

sir but

how can we assume that the string unwounds. in these type of pulley questions the amt of thread is constant.

bvsatyaram

The solution problem turn extremley tedious had the length of the string been constant... In this case, as the disc rotates, the point to which teh string is tied comes down, so the string is no more horizontal... then the tension is not horizontal.... going on and on... the problem turns in to a huge demon... i tried to solve this way, when i was preparing for JEE, but never solved it successfully.

In any problem like this, unless other wise specifically stated, u can assume that the sring is wound around the disk.

"Strings tied to fixed point" problems can be solved only in linear motion...

priyesh

ok

thanks sir

rajat.khanduja

I stand for 2v/r

By definition of pure rolling, the velocity of the lower most point is equal to the velocity of the surface in contact ( and not always zero as thought by most). It is zero only when the rolling is on the ground and then the velocity of the lower most point is zero as it is the same as that if the point of contact on the ground.

So, here,

rw- v = v (same velocity by constraints)

r = 2v/r

PS : rate me if I'm correct

kislay

satyaram sir is correct.....................

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