# Torque, point or axis of rotation

1. Feb 23, 2013

### bgq

Hi,

1) Do we calculate torque with respect to a point or with respect to an axis?

I have read them both in different resources, and so I am confused!!

2) If we calculate torque with respect to an axis, many introductory textbooks discuss the motion of the gyroscope by considering how the torque affects the angular momentum, but there is a problem that the angular momentum is specified with respect to a different axis of rotation than that of the torque (The angular momentum is about the axis of the wheel, while the torque is about the axis passing through the pivot and perpendicular to the first one); how can we resolve this?

Thanks to any helps.

2. Feb 23, 2013

### Simon Bridge

In 2D, a torque is about a point.
In 3D it is about an axis.

Just the same way that a rotation is about a point in 2D and an axis in 3D.
But note: angular momentum and torque are not like regular vectors.
per your example, the same texts should show you the math of how the torque affects the angular momentum.

3. Feb 24, 2013

### bgq

My problem is that when applying the formula T = r × F in 3D, r is determined with respect to a point where there are infinite axis of rotations passing through this point, so T is determined with respect to which axis?

4. Feb 24, 2013

### Simon Bridge

... the axis of rotation of course... which is a vector perpendicular to both r and F... points in the same direction as the pseudovector T. That is what the direction part of the cross product is for.

5. Feb 24, 2013

### bgq

What is meant by pseudovector?

6. Feb 24, 2013

### Simon Bridge

7. Feb 24, 2013

### bgq

Thank you very much.