Torque Question

[boysetsfire]

A wheel free to rotate about its axis that is not frictionless is initally at rest. A constant external torque of 50 Nm is applied to the wheel for 20s, giving the wheel an angular velocity of 20pi rad./s . The external torque is then removed and the wheel comes to rest 120s later. Find a) the moment of inertia of the wheel and b) the frictional torque which is assumed to be constant.

part a) I have T = I * ( angular acceleration ) heres where im not sure what to do, the book has the answer as I = 15.9 which works if you dont take into account the torque caused by friction. So is the book wrong or for some reason you dont have to take into account the frictional torque?

[darkbob5150]

I think you might be right to be suspicious...unless by external torque they mean that the TOTAL torque acting on the wheel is 50Nm.

The total torque acting on the wheel SHOULD be given by Applied Torque - Frictional Torque. The Applied Torque is 50Nm like they say, the Frictional Torque can be worked out and should be I x Pi / 6. The Total Torque is then equal to I x (angular acceleration) as you said. That comes out to give an I of 13.6 kgm^2.

Not sure - might just be a dodgy question/answer!

Let me know if you work it out...