# Stability of a bicycle

## Question:

We can keep our balance on a moving bicycle, but then why it is nearly impossible to balance on a stationary bicycle?

This one is a case of the conservation of angular momentum.

Any rotating object, in this case your bicycle wheels, has a property called angular momentum in just the same way that an object moving along has linear momentum. Just as a moving body requires a force to change its linear momentum so a rotating body requires a turning force (or torque) to change its rotation. You will know that linear momentum is a vector, well angular momentum is also a vector and in this case it is the direction of the axis of rotation that requires the force to change it. (The rate of rotation will also require a force but that is irrelevant in your question).

Now a stationary bike wheel has no angular momentum and so does not need a force to change the direction of the axle – in other words the bike can easily fall over.

However the rotating bike wheel has angular momentum and so requires a force, in some cases a considerable one, to make the axle of the wheel change direction, and so the wheel stays upright.

If you can get a bike wheel separate from a bike try tying a piece of string to one end of the axle, spinning the wheel and then seeing if you can hold the wheel up by this string – it should work. This is also related to the properties of the gyroscope.

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