h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

What an interesting article! It reminds me of happy hours playing with Eiffel Towers and spinning gyroscopes as a kid.

One minor point: angular (and linear) momentum is not technically conserved. I'll stick with the linear momentum case, as it's easier;

Impulse = change in momentum.

Impulse is the Force applied multiplied by the time it is applied for.

In symbols,

Ft =mv2 - mv1

F is force applied, m is the mass of the object the force is applied to, t is the time for which it is applied. v2 is the velocity of the object after the force is applied, and v1 is the velocity before.

So forces create and destroy momentum all the time.

The important thing about pushing a gyroscope over is that you apply a force perpendicular to the angular momentum (ie. the axis about which the top is spinning). The gyroscope responds with a force that works to return the axis to the vertical, countering the applied force, and a precession force, which you describe. These two forces arise from the angular version of the formula above, thus reducing the angular momentum of the gyroscope. So pushing a gyroscope over (or the effect of gravity doing the same thing) slows it down.

Deano

Momentum is conserved! This is not the same to say momentum cannot be created. To be conserved, momentum must be created in two different bodies, with opposed orientation. The same force that creates momentum in some body, by Newton's third law, must be countered by another force in the other body, which creates the exact same amount of momentum, in the opposite orientation. The total momentum remains the same, since the new momenta created cancel each other when added.
If not, by increasing the momentum of any body, its energy would be increasing, and energy also must be conserved.
Astrônomo