Necessary Centripetal Force
Any object that moves in a circle requires a force directed towards the center of the circle to keep it moving in a circle. This force is known as the centripetal force. For example,
- The earth moves around the sun because of gravitational force.
- A roller coaster can move in a circular loop because of normal reaction exerted by the tracks on the car.
- A race car can move in circular track because of friction between the wheels and the ground.
In these examples, the necessary centripetal force is provided by gravitational force, normal reaction, and frictional force respectively.
As a top spins, its center moves in a circle around a vertical axis. Which force provides the necessary centripetal force for it to move in a circle?
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1 solution
The only forces acting on the top are weight, normal reaction and friction. The center of mass of the top moves in a circle that in parallel to the ground. The necessary centripetal force is directed towards the center of the circle.
We see that the weight of the top and the normal reaction by the ground on the top are both perpendicular to the top, and frictional force is in the plane of the ground. Therefore, the only force that can cause the center of mass of the top to move in a circle, that is to precess is the frictional force between the top and the ground.