# Problem in choosing axis in rotational mechanics?

1. Oct 29, 2011

### vkash

In rotational mechanics i am mostly confused with the axis of rotation. Let me give you some examples of question.
about which axis. Is that instantaneous axis of rotation.
Now once again. which axis should i choose.
IAOR(instantaneous axis of rotation). I can;t find that.
However when i see solution of this example they take torque around the center of the rod and solve it. Why they take center of the rod. Why not any other point. If any other point on the plane is taken then answer may vary.
this point is really confusing me in rotational dynamics. which axis should i choose when i do questions.

I think general physics is correct place for this question.

2. Oct 29, 2011

### Curl

angular velocity is a vector, and it doesn't matter what axis you chose, it will be the same.

3. Oct 29, 2011

### vkash

In second question, Take axis around point where rod touches the surface.
$$I=\frac{ml^2}{3}$$
$$T(Torque)=mgl$$

$$T=I\alpha$$
$$\alpha=\frac{T}{I}$$
$$\alpha=\frac{mgl}{\frac{ml^2}{3}}$$
$$\alpha=\frac{3g}{l}$$
Now take the center as the axis. Your answer will $$\frac{6g}{l}$$. That's different.
How will you explain this with our answer.

4. Oct 30, 2011

### Curl

If you take torques about the CM, then you must also account for the torque by the ground (the normal force).

If you sum torques about the ground, make sure you compute the inertia about the end of the rod. Use the parallel axis theorem.