A small boy lifts a
and precesses on a rotating platform as he does so
Laithwaite's demonstration the
boy can not hold the gyroscope by
the end until it is spinning, he
then is able to lift the gyroscope by the end. Whilst doing this he is
standing on a rotating platform and precesses.
The moment of the weight of the
gyroscope is balanced by the change in moment
of momentum as precession
occurs. This means that the weight of the rotor experienced by the boy
is the similar to that if he had picked the rotor up from the middle of
the shaft as opposed to the far end. This he would have been able to
do he could just not hold the rotor by the end of the shaft due to the
moment. A free body diagram of the stationary rotor and a simplified
representation of the boy's hand
is shown in the following diagram.
Forces acting when a stationary
rotor is picked up from the end of shaft
is a further explanation of the motion in the following
Demonstration using a
demonstration was repeated using a bicycle wheel as the rotor and
standing on a platform that is free to rotate. It is noted that the
mass and moment of inertia
bicycle wheel are less for the bicycle wheel than that for the rotor in
Laithwaite's demonstration however the principles behind the motion are
can be seen that
when the bicycle wheel is tilted upwards precession occurs. This is due
to conservation of moment of momentum. When the bike wheel is
horizontally the moment of momentum about the vertical plane is zero,
when the wheel is held upwards some of its angular momentum is
transferred to the vertical plane, since moment of momentum must be
conserved (assuming friction has minimal effect) the platform spins in
the other direction.
Platform spins in other
direction to the the rotors vertical component of spin to conserve
moment of momentum
the moment of momentum of the bike wheel still has a component in the
horizontal plane when precession occurs the spin axis of the bike wheel
changes direction and therefore so does direction of the moment of
momentum of the wheel.
this change in moment of momentum introduces a couple
Q. This couple is
seen to be in the opposite direction of that due to the weight of the
bike wheel and so reduced this, making the wheel easier to hold.
The precessing rotor viewed from
above with couples marked on
the wheel is held downwards moment of momentum must again be conserved
and so the platform spins in the other direction.