It is important to grasp the idea that, to keep a body moving in a circle there must be a force on it directed towards the centre. This is called the centripetal force. Before Newton’s time it was believed that invisible spokes radiated out from the sun and pushed the planets round. Newton’s insight into the problem convinced him that a push such as this was not necessary. The planets, carrying their atmospheres with them, go on moving in their orbits because the great vacuum of space offers no opposing force to their motion. Centripetal force is, however, required to produce the continuous change of direction which occurs in the orbit and this is provided by gravitational attraction.
We can try a simple experiment to demonstrate centripetal force by securely tying a suitable mass on the end of a string and swinging it round. The pull in the string which is providing the centripetal force can easily be felt and we notice that it varies according to·mass, speed, and path radius. In a laboratory experiment, of course, the circular motion of a mass on a pivoted arm will, if left to itself, rapidly come to rest owing to air resistance and so on. No such resistance is offered to the planets as we have already said; so they continue to move.