Curious Case of the Spinning Flywheel Briefcase

[louie3006]

Q # 1. the film crew CANDID CAMERA replaces person's briefcase with an identical one that contains a mounted and spinning flywheel. explain what happens when the person tries to carry the briefcase around the corner?

 

[dynamicsolo]

This is quite an old practical joke, which has probably been pulled on someone at every technical university since a gyroscope and power supply could fit in a briefcase...

You know that angular momentum is conserved: this means that the spinning flywheel will try to maintain the orientation of its rotation in three-dimensional space. Suppose that the flywheel is mounted vertically in the case, so that it is spinning around a horizontal axis. Further suppose that the person walking with this case is headed westward down a corridor and that the direction of the flywheel's angular momentum is pointing northward.

The person now goes to turn a corner to walk, say, northward. This would have the effect of attempting to turn the flywheel so that its angular momentum would point eastward. Needless to say, it won't just do that. So if the person is applying a torque to try to turn the angular momentum to the east, how will the flywheel respond?

 

[louie3006]

it will resist the change and try to stay in its original position, am i right ?

 

[dynamicsolo]

it will resist the change and try to stay in its original position, am i right ?

You're right, in that the flywheel will try to keep its rotation axis pointing north. But the person carrying the briefcase is more than strong enough to force the change. The result is a sort of compromise: the person, we say, produces a torque on the angular momentum of the flywheel, causing the direction of the angular momentum to pivot at right angles to its current direction and the person's applied force. Since the angular momentum was pointing north and the applied force is eastward, the torque on the flywheel will point toward the floor, so that the flywheel will orient itself into a horizontal plane, causing the briefcase to twist violently in the person's grip. (If you're not expecting this, it's rather alarming...)