Forces - Circular Motion

Circular motion can be defined as a movement along a circular path. Newton's first law of motion states that a moving object will travel in a straight line at a constant velocity unless a force acts on it.

This means that in circular motion, there must be a force constantly acting on the object as it is constantly changing direction. This force is given the name centripetal and is a resultant force of the motion and whatever is causing the circular motion. Centripetal force is a virtual force and is provided by the action of something else.

In the case of the ISS, the Earth's gravity is responsible for the centripetal force. Be careful not to confuse centripetal force with centrifugal force - they act in opposite directions. Fairground rides are very good examples of circular motion, the centripetal force is provided by the framework of the ride. It is the centrifugal force that feels like it is trying to throw you off the ride that adds the excitement. There are many other examples of circular motion that are used in senior high school; vehicles going round a corner, a planet orbiting the Sun, a satellite orbiting a planet and throwing a discus or a hammer at an athletics event crop up quite often. In the senior high school exam, you need to be able to identify what is responsible for the centripetal force in specific examples and also what factors affect the magnitude of the force.

The magnitude of the centripetal force depends on the mass of the object that is moving in circular motion, the speed of the object and the radius of the circle in which it is traveling. Increasing the mass of the object will mean that the centripetal force required to keep it in circular motion will be greater. If all other things are equal, an object that is traveling with a greater speed will require a greater centripetal force to hold it in place. Unlike the other two factors, increasing the radius of the circle in which an object is traveling will actually decrease the centripetal force required to hold it on track. You may even be asked to explain why these factors affect the force in the way that they do.

The final thing to remember about circular motion is that it is an acceleration. Since the object is moving along a circular path, its direction of travel is constantly changing. This means that its velocity is also constantly changing (remember velocity is motion in a specific direction, it is a vector quantity). By definition, acceleration is a change in velocity so an object following a curved path must therefore be accelerating. Its speed remains the same.