# Torque and rotational motion

What happens if the torque is perpendicular to the axis of rotation or torque is in the direction of rotation?

• What do you mean by "torque is perpendicular to the axis of the rotation" ? Do you mean force ? – Jaswin Feb 14 '15 at 18:19
• given a force and a reference point you can define torque, which is $\vec{r} \times \vec{F}$. – Jaswin Feb 14 '15 at 18:21
• Could you draw a picture of what you mean and attach it here? – Kyle Kanos Feb 14 '15 at 18:23
• i meant any component of torque perpendicular to the axis or any forces parallel to the axis. – Amaljith V Nair Feb 14 '15 at 18:23

## 1 Answer

The change in angular momentum of an object is equal to the integral of torque over time:

$$\Delta \vec{L} = \int \vec{\Gamma}dt$$

Note that I wrote both angular momentum and torque as vectors. When torque is along the axis of rotation, you get either an acceleration or deceleration of the angular motion (depending on the relative direction of each). When torque is at right angles, you get precession, such as happens for example in gyroscopes. When torque is partially aligned, you get a bit of both. This is nicely explained at this link.