During childhood majority of the kids have taken a ride in some or other kind of a Merry-Go-Around. Some are very huge and fanciful run by electric motors, while some are very small rotated by one of the children who climb back on the swing after giving it a lot of push. The swing then starts rotating around its center and stops after some time on account of friction in its bearings. We will talk about the force of friction in another blog to understand Physics happening in it.
But where is Physics encountered in the rotation of the merry-go-round and how do children experience it is a moot question. Well, we can understand Physics by considering that when a mass tied to say a string is subjected to a circular motion. So, to remain in the position undergoing uniform circular motion, the force which keeps the mass maintaining its rotational position is the Centripetal Force. This force is again a vector having a magnitude and direction, which is directed radially towards the center. But we also know from the 3rd law of motion, that for every action there is an equal and opposite reaction. So, the force opposite to the Centripetal Force is called the Centrifugal Force and acts outwardly away from the center of the rotation and keeps the mass in its place undergoing the uniform circular motion attached to a string. The image below explains it.
The Centripetal Force Fc = mv²/R — — — — — — — — — -(1)
Let us now go back to the example of the merry-go-around where if a child tries to jump off the merry-go-round in a direction other than the direction the swing is rotating, what will the child experience. This can be understood say by an example. Let us assume the swing is rotating in a clockwise direction. But the child jumps out of it, facing an anti-clockwise direction, and vise versa. Such a child experiences a sudden force that may make him/her stumble if he/she is not prepared for this sudden experience of a force that topples him/her.
The key force acting when anything is rotated in a circle is given by the Centripetal Force. This force kicks in no sooner than anything is rotated having a mass. Recall, we have already realized the fact that Velocity is a vector quantity (refer to my blog PHSK-5) that has both a magnitude and a direction. So, if the swing is rotating at a uniform speed, the child sitting on it has a magnitude of the speed but also with every second as the rotation is taking place, the “direction of the velocity vector” keeps changing. In other words, the velocity keeps changing every moment. As described in the 2nd image above.
Also recall that rate of change of momentum as per the 2nd law of motion is the applied force. Where momentum (mv) is a vector and product of mass (m) and velocity (v). So, as the swing is rotating, the direction vector of the velocity is changing every moment. This velocity vector is tangent to the circumference of the circle in which the child is enjoying the rotation. Hence when a child jumps out in a direction different to that of the direction in which rotation is taking place, there is a change in velocity vector which results in a sudden application of force on the child by it he/she stumbles.
There are two more things which school-going children may discover and have an influence on them practically daily as a result of the circular motion’s principles of Physics. First is the “washing machine” used at home to wash clothes. Its rotational motion ensures a majority of the water is drained out before clothes are put in the dryer. It will amuse the children that the same principle is applied to collect the fission grade pure Uranium through centrifugal machines. The “m” values play the key role in equation (1) above to collect the fission grade Uranium, as the atomic weight of fission grade Uranium is unique like of any other chemical element existing on this earth. While the second is the effect of the rotation of the earth on its axis which makes us experience the change in the direction of wind movement and experience hurricanes. This is a result of the Coriolis Force and requires a separate blog dedicated to it.
May 02, 2021