1. The problem:(adsbygoogle = window.adsbygoogle || []).push({});

A Force F is provided at the top of a rod placed vertically placed on a smooth frictionless surface. The length of the rod is L and its mass, M. Find the angular acceleration of the rod at the instant the force is applied.

- Please tell me how to solve this and why you have solved it in that way.

2. I solved this problem about the centre of mass, but can it be solved about any other point?

3. If I can solve it about any point I wish, why so?

4. Is it necessary that a body such as the one in the above problem must rotate about its centre of mass only? How can we state this? What is the proof? How do we find out about which point the problem must be solved.

5. I don't know if my method of solving was correct, but when I solved it about the centre of mass I got one answer, and when i solved it about the point in contact with the ground, I got a different answer. So, is angular acceleration different about different points? Or is angular acceleration an intrinsic property? If so, why? Are the answers I get (by solving in the aforementioned two ways) the angular accelerationrelativeto the respective points about which I have solved the question? If they are, how do I get the answer in the ground frame? What would have been the pseudo forces if it was relative acceleration I was calculating?

I solved the problem in the following ways:

a) About the centre of mass:

F*(L/2)=I*angular acceleration=(M*L^2)/12*angular acceleration

=>angular acceleration=6*F/(M*L)

b) About the point at the bottom of the rod, in contact with the frictionless horizontal surface:

F*(L)=I*angular acceleration=(M*L^2)/3*angular acceleration

=>angular acceleration=3*F/(M*L)

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# Homework Help: Angular Acceleration - Should an unpivoted rod rotate only about the centre of mass?

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