Gyroscopes can be very precise navigational tools. If they are well-balanced and there is no gravitational torque (or torque from anything else) then a gyroscope with a particular angular momentum should maintain that direction. A rotating gyroscope in a plane, for instance, will keep pointing in the same direction even when the plane changes direction.

Toy gyroscopes are generally built so that there is a torque from the force of gravity acting on the gyro. This is a good example of impulse in a rotational setting. A gyro with an angular momentum that is anything other than vertical will feel a gravitational torque directed horizontally, perpendicular to the direction of the angular momentum.

According to the impulse equation, ΔL = τ Δt, the torque will produce a change in the angular momentum, with the change in momentum and the torque pointing in the same direction. This has the effect of rotating the gyro's angular momentum about a vertical axis - this is known as precession.

We'll add a mass to our precessing bicycle-wheel gyroscope at about the point where the cut string was. What does the gyroscope do?

1. start precessing in the opposite direction
2. precess the same way, but slower
3. precess the same way, but faster
4. there's no change in the precession