Kim Robertson, 10, of Richmond, Va., for her question:

WHAT ARE CENTRIFUGAL AND CENTRIPETAL FORCES?

If a ball is whirled at the end of a string in a circular motion it would seem as if the ball were suddenly very heavy and were pulling harder as it whirled faster. The force of the string pulling the ball toward the center of the circle is centripetal force, while the force seeming to act on the string is called centrifugal.

If you let go of the string neither force would work, and the ball would continue to move in a straight line with constant speed.

Physicists call a merry go round a rotating reference frame. You do not need the idea of centrifugal force if you observe your motion with respect to the ground instead of to the merry go round. You would have to say the merry go round exerts friction between your shoes and the platform. This keeps you from moving in a straight line with a constant speed.