Can anyone help me answer these moment of inertia questions please?

[sowmit]

The question says:
Imagine a solid disk, made of uniform material, a radius R and thickness L. What is the ratio of L/R if the moment of inertia of this disk about the axis passing through the center and perpendicular to the plane of the disk is the same as the moment of inertia about the axis passing through the center and parallel to the disk? [I definitely don't know how to answer this. I don't think I even understand how to start it off]

2nd Question says:
Is it possible to have a torque on an object while the net force applied to it is zero? If not, why not? If yes, give an example.

-> I said it is not possible because torque= Force * Sin(theta). Since force is 0, torque is o as well.

Can someone help me with these questions please?

 

[weesiang_loke]

Hi,
for the first part: the moment of inertia of the disc passing through perpendicularly to the surface of the disc at its centre is (1/2)*M*(R^2). and the other case the moment of inertia is (1/12)*M*(3R^2 + L^2).

http://en.wikipedia.org/wiki/List_of_moments_of_inertia

so just compare and you will find the ratio.

for the second part i am not sure about the answer.
But if the vector sum of the applied force is zero, for example rotating an object, the torque is not zero and is commonly known as 'couple'