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Mechanics

Blazing goIITian

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19 Feb 2007 18:27:10 IST

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490

Circular disk

None

A force of variable magnitude is being applied on a solid disk tangential to it at its topmost end.Assume high friction at all surfaces.Mass is 'M' and radius is 'R'.

F(t) = at + b (a & b are constants)If the disk is at rest at t=0,

Q1. Find the speed 'v' after 5 seconds of center view

(a) 10(5b + 2a )
      ---------------
         3MR

(b) 10(5a + 2b )
      ---------------
         3MR
(c) 7(5a + 2b )
      ---------------         [ans:- (b)]
         3MR

Q2. Find A_cm as a function of time

[ans:- 4(at + b)   ]
         --------------
            7M

I'm getting correct ans for Q1 . Then I put A_cm= dv/dt but Im getting different ans.
My ans--4/3M (at + b)

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Comments (7)

avdesh100

Blazing goIITian

Joined: 9 Feb 2007

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19 Feb 2007 20:05:53 IST

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ur answer is correct

Moderator

Joined: 19 Oct 2006

Posts: 961

20 Feb 2007 11:54:38 IST

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Dear Neeraj,

Experts answer only one question at a time. Please let us know which of these questions you want to be answered first and then post other queries on a new page

~ forum moderator

Neeraj Agarwal

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Joined: 22 Jan 2007

Posts: 2039

20 Feb 2007 11:56:23 IST

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See...both Q's are related ...I said I got the first right and then used integration to get 2nd ans.....but it didn't match.

For me , just answer Q2

Neeraj Agarwal

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Posts: 2039

20 Feb 2007 11:58:43 IST

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I request you to please tell me how to post figure with Q's

coz once I posted...and I couldn't find it afterwards

Moderator

Joined: 19 Oct 2006

Posts: 961

20 Feb 2007 14:48:47 IST

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Dear Neeraj,

We will have your Q2 answered.

~ moderator

vishak p

Blazing goIITian

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20 Feb 2007 14:50:34 IST

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moderator sir \ madam , i am not gettign any replies !! please pass my question to the experts !! pleassse !!

Bipin Dubey

Forum Expert

Joined: 23 Jan 2007

Posts: 7942

21 Feb 2007 20:49:00 IST

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When you apply force and torque equations it gives
2F = (M + I/R²).A_cm
For disk I = MR²/2 (negligible thickness)
Hence it gives
2F = (3M/2).A_cm
=> A_cm = 4F/3M
=> A_cm = 4(at+b)/3M

There may be a printing mistake.You have done correctly.

Best Wishes

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