# Rotation of a body around which axis

1. Apr 26, 2012

### jd12345

When i apply torque on a body, it will rotate around whcih axis? I guess its the axis along centre of mass but i don't know why?

Also if the answer is centre of mass what will be the orientation of the axis- I mean there are infinite lines passing through a single point so which of those axis will be the one around which body rotates?

2. Apr 26, 2012

### Staff: Mentor

You can always describe the movement of a rigid body as a rotation around its center of mass (with a certain orientation of the angular momentum) plus a velocity of the center of mass (in some direction).

If you apply a torque without any net force (which means that the sum of forces (not torques!) is 0) to an unmoved object, it will begin to rotate at the position where it was before. If you apply a torque with a net force (for example, kick one side of the object), it will do a combination of rotation and global movement in the direction of the kicking force.

The same as the axis of your torque (F x r).

3. Apr 26, 2012

### phyzguy

This statement is not true in general. An irregularly shaped body is described by a tensor of inertia, which means that the body will not necessarily rotate around the axis defined by the torque. The two axes will only coincide if the torque is applied along one of the principle axes. Otherwise, the angular momentum will be along the axis of the torque, but the angular velocity will be around a different axis, as described by the equation L = I w.

4. Apr 27, 2012

### Staff: Mentor

I meant the axis of angular momentum with this reply.
Maybe I should have said this explicitly.