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).
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.
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
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.