In 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.
Explanation of motion
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
There is a further explanation of the motion in the following demonstration.
Demonstration using a bicycle wheel
The 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 of the bicycle wheel are less for the bicycle wheel than that for the rotor in Laithwaite's demonstration however the principles behind the motion are similar.It 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 held 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
Since 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.
Therefore 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
When the wheel is held downwards moment of momentum must again be conserved and so the platform spins in the other direction.