# Gyroscopic Precession Question

1. Apr 12, 2014

### Lego Science

Veritasium posted a video on his YouTube channel called "Gyroscopic Precession." () In the video he explained how spinning the wheel caused the torque tho point toward "the camera" at 90° to the angular momentum. My first question is how and why does the torque point perpendicular to the wheel. My second question is why the torque force points to "the camera" and not the opposite direction, to me it seems like either direction would be kind of the some to the wheel. Please elaborate.

Last edited by a moderator: Sep 25, 2014
2. Apr 12, 2014

### paisiello2

The torque is a rotation about an axis and the axis is perpendicular to the wheel.

The reason the torque vector points to the camera and not the opposite direction is pure convention. It could easily be the other direction as well and nothing would change as long as you are consistent throughout with whatever convention you pick. In this case he used the well established convention of the "right hand rule".

3. Apr 12, 2014

### Lego Science

Thank you paosiello2. As I have done more reading I have thought that maybe the reason the torque points toward the camera is because the wheel is spun around the rope holding the wheel, and the torque favors the centripetal force direction? I also have another question: The wheel spun counter clockwise (from the front) and the wheel spun around the rope counter clockwise, is this always a direct correlation?

Last edited: Apr 12, 2014
4. Apr 12, 2014

### paisiello2

No, it is only a convention. It could easily point away from the camera as well. As long as you suddenly don't change the convention you adopted from the start then you will always end up with the same answer.

The centripetal force direction is not really the issue. It's more the fact that you have a rotation vector of the wheel being added to the rotation vector of gravity acting on the wheel.

Yes, there is always a correlation. Here is my understanding of what is going on:

Gravity creates a torque about one axis. As the wheel starts to swing down caused by this gravitational torque, the spin of the wheel creates a restoring torque that momentarily also causes a slight rotation about the axis of the rope. As long as this gravitational torque acts on the spinning wheel then their will be this rotation about the axis of the rope. This rotation is technically called a precession.