IIT JEE , AIEEE Forum - Physics , Mechanics

garima4

A body rotating at 20rad/sec is acted upon by a constant torque providing it a declaration of 2rad/s².At what time will the body have kinetic energy same as the initial value if the torque continues to act ?

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EDIT: if the body is held by an axle or somethin..
the only source of kinetic energy is rotational energy. since

is decreasing, kinetic energy decreases also..
so, the answer is - it is never possible..

unless, there is some translational KE which accounts for loss in rotational K.E..

computer001

there is only 1 force acting...hence the force which brings the body rotational deceleration will obviously cause the body to have translation motion..

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oh i was under the impression that it was hinged by an axle or something..

computer001

1/2 *I*w₁²= 1/2 I w₂² + 1/2Mv^2
w₂=w₁+ @t
v=a*t
a=r@
v=rw solve

joyfrancis

The body is just rotating .

So for initial KE to be equal to the final KE |w_i| = |w_f|...obviously one will be clockwise and the other will be anticlockwise so w_i = -w_f.

since is const, we can apply eqn of motion

.: wf = wi + t

t = (w_f - w_i) /

= 2w_i/

= (2(20))/2 = 20 seconds....*ans*

computer001

its given a rotating body is acted upon by a torque:
tht means initially body is havin only rot motion...where is it saying...it always has only rot motion???

the q says deceleration...
once the w becomes <0 the force causes acceleration in other direction...it wont be 'deceleration' ne more

joyfrancis

If the body is only rotating and not translating initially then there has to be some kind of pivot, that means it is fixed in one position and can ONLY rotate, the force or torque applied on the body will reduce the angular velocity...translation is NOT taking place.

spideyunlimited

ARRE BHAI JOY is right!
we have to find time after which rotational speed is equal in magnitude but opposite in direction. that is when KE will be equal

spideyunlimited

A body rotating at 20rad/sec is acted upon by a constant torque providing it a declaration of 2rad/s^2 . At what time will the body have kinetic energy same as the initial value if the torque continues to act ?

v = u + at
---> w2 = w1 + @t
-w1 = w1 + @t
-20 - 20 = -2t
-40 = -2t
t = 20 seconds

joyfrancis

aeee mere makre dost...thanks for the rate...tune bhi same hee cheez likh dee :P?..shabaash!!!...le 2 number!!

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