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11 Feb 2009 00:14:56 IST

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please try to solve interesting

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	A disc of mass 2M lying in vertical plane is free to rotate about a fixed horizontal axis passing from its centre. A particle of mass M is tied to a string of length 2R and another end of the string is attached to the disc as shown in figure. If particle is released from shown position, then the angular speed of the disc as string becomes taut will be

	1.

	2.

	3.

	4.	none of these

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Comments (13)

Ankit Rana

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11 Feb 2009 00:49:52 IST

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is the answer root (g/r)

negative marking

Blazing goIITian

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11 Feb 2009 00:53:45 IST

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yup....must be..just conserve energy..its (g/R)^1/2

JPS

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11 Feb 2009 01:07:25 IST

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is the answer a sqrt g/r

saharsha kumar keshkar

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11 Feb 2009 16:43:35 IST

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i think, first option is correct>>>>

Aatish Pandit

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11 Feb 2009 17:16:40 IST

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well i think we shud conserve energy.......

mgR = (1/2)mv^2 + (1/2) I w^2

at the instant when string becomes taut...v = Rw...

solving we get w = root(g/R).....

shraman asa

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11 Feb 2009 17:17:48 IST

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yep itz root(g/r)

IITAIEEE

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12 Feb 2009 00:03:00 IST

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Correct Answer: 2 Solution:Ans. (2)

Rohit Khattar

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12 Feb 2009 01:54:34 IST

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hi...see first of all i feel tht the disc will nt rotate untill there is an external torque...now since the only force tht is present tht will provide torque is tension which will act only after the string gets taught....so as soon as the string gets taught the angular velocity shud be 0...so pls. explain tht y r u all applyin energy conservation...pls. correct me....

Gaurav |spideyunlimited| Ragtah

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12 Feb 2009 13:35:24 IST

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how do we get (2) ??

Gaurav |spideyunlimited| Ragtah

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13 Feb 2009 15:48:27 IST

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Arre koi batao? Even I used the same conservation method, but how is the answer (2)?

pizza6666

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13 Feb 2009 16:20:13 IST

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I am getting option 1Did anybody get the solution???

shilpa nair

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13 Feb 2009 16:42:01 IST

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@spidey.... rohit khatta is rite i suppose....the disc will not even roll till string becomes taught...sobyu using energy princv = (2gr)^1/2 where v=vel of dat falling thng...now just after disc will start revolving...v=rw now get w from here which is 2nd option!

Gaurav |spideyunlimited| Ragtah

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13 Feb 2009 20:02:32 IST

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so they have ignored 1/2 I w^2

and shilpa:

rw = sqrt(2gr)

gives 1st option. not 2nd.. This questions is flawed, methinks.. lol

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