Rotating Platform

An object placed on a rotating platform experience a force directed away from the centre known as the CENTRIFUGAL FORCE. All of us feel this force when we are on a rotating system like a merry-go-round or on a rotating platform like the one we have here. On account of this force a ball rolled or thrown to another person on the rotating platform traverses in a curved path. The curvature depends on the speed of the ball as well as that of the platform. Also, the ball curves away in a direction opposite to the direction of the rotation of the platform.

Here, we have a chair fixed to a rotating platform. The chair can rotate freely about a vertical axis. The chair is fitted with rod and sliding weights. After you sit on the chair, move the weights as far away from you as possible. Now, set the platform to rotate slowly. While it is rotating, bring the weights closer to you. The platform speeds up its rotation. Again, on moving the weights away, the platform slows down. This is due to the conversion of angular momentum. Suppose a body is rotating at a certain speed. Now, if the radius of the object is reduced, the speed of rotation increases by the same factor. This is clearly evident in several situations. A dancer or a skater folds the hands very close to the chest while pirouetting. As soon as the hands are stretched out, the speed of rotation perceptibly decreases. Ballet performances are replete with such motions.

Now , the moot question is why should the rotation speed when the radius of the mass is reduced ?
Well. All rotating bodies posses what is known as rotational inertia. It indicates the ease or the difficulty with which one can set a body to rotate. This property is related to the distribution of mass of the body. If all the mass is compactly organised, the rotational inertia will be less, implying it can be rotated with a small amount of force. On the other hand, if a body possesses same amount of mass but the mass is distributed over a large area, its rotational inertia will be higher. That is, one has to apply a greater force to rotate it. Suppose we have a system where in the mass distribution can be changed. Let us assume that the body, initially in a spread out orientation, is rotated by applying a certain force. If the body now contracts even as it is spinning, its rotational inertia is reduced. Therefore, a larger part of the force originally applied goes into spinning the object. Hence the speed of rotation increases. This effect is not limited to dancers and skaters alone. Springboard divers use this idea to perform the breathtaking somersaults they do. Even though the divers spend energy to perform half a turn somersault, they execute one and half or even two turn somersaults. Obviously they have to spin their body very fast. In order to achieve this, they pull their arms and legs towards the centre of their body. This considerably reduces the rotational inertia enabling the diver to spin faster and perform one and half or two turn somersaults.