Question: How does a gyroscope work? Eg. On a bike?

Daniel Scully answered on 22 Nov 2011:

Once a heavy object has started moving, it’s very difficult to slow it down.
You have to put a lot of effort and force into slowing it down because it has a high INERTIA.
When an object has inertia, it likes to keep moving in the same way and you have to put in a lot of effort to overcome that.
If the object is moving in a straight line, then this is LINEAR INERTIA.

In the same way, things that are rotating or spinning have a ROTATIONAL INERTIA.
This inertia means that if it’s already spinning it wants to keep spinning, unless something tries hard to stop it.
The rotational inertia also means that it also want the axis around which it rotates to stay the same.
So spinning things like to stay spinning in the same direction.

This is why your bike is more stable when your moving than when it is stationary – your wheel is spinning and so has rotational inertia. The axis it’s spinning on is horizontal, and it doesn’t want that to change, so you stay upright.
When you stop and your wheels are no longer spinning, there’s no longer any rotational inertia and you fall over (unless you put your feet down).

A gyroscope works this way too.
In the centre of a gyroscope is a disc. The disc has very little friction and is heavy so almost any vibration or motion will set it spinning. When it is spinning, its rotational inertia keeps it spinning in the same way.

Usually there is a small weight which encourages it to start spinning so that it’s axis of rotation is pointing vertically.

You then put this spinning disc into a frame so that when the device it’s mounted on (like your bike) moves around, the disc can still spin how it wants.
(the wikipedia page has a good diagram of one of these ways: http://en.wikipedia.org/wiki/Gyroscope)
Depending on where you’re using the gyroscope you could also float it.

The important thing is the disc can move however it wants regardless of how the thing it’s attached to is moving. But because of its rotational inertia, it likes to keep spinning with the same axis, which you encouraged to be vertical.

So, when you are going up and down hills on your bike, and leaning into corners, the disc in the gyroscope keeps spinning with its axis vertical… and there you have it… a device which tells you which way is up.

(Just to be clear, the disc’s axis of rotation will change a bit, particularly if you make sudden or violent movements, but it will move much less than the device it’s attached to)

This all works much better than just hanging a heavy weight from a string – then there is little to stop the weight swinging, at which point it’s useless. It’s the resistance to changing the rotation axis from the rotational inertia that makes it all work so well.