Imagine you are
twirling a ball on a string around your head.
If the string suddenly broke the ball would go flying off in a straight
line. The string then, is acting as
some kind of force that we will call the centripetal force.
The centripetal force on an object moving in a curved path is the
inward force that must be exerted to produce this motion. It always acts toward
the center of curvature of the object's path.
The earth revolves around the sun. What keeps the earth from flying away? The force of gravity of the earth and the sun provide that centripetal force.
The equation for centripetal force is;
Examining this equation we note that increasing the mass or velocity increases the centripetal force required to maintain the circular motion. Decreasing the radius will also increase the centripetal force required to maintain the circular motion. This can have some important consequences for driving a car.