Life in a non-inertial frame

Frames of Reference: Playing ball in a non-inertial frame of reference page 2

   To observe a thrown ball from the point of view of a non-inertial (accelerating) frame of reference, we must place ourselves in such an acclerating frame of reference.

   We could, for example view a ball thrown horizontally from an accelerating car or bus. This is clumsy, because, first of all we would soon be out of sight of the ball; furthermore if we kept on accelerating, we would soon be traveling dangerously fast.

   There is an unexpectedly simple way to be in an accelerating frame of reference. This stems from the fact that a point on a rotating object moves in a circle, and to move in a circle, you have to be continually accelerating.

   You are probably familiar with the fact that to keep an object in circular motion, a force toward the center of the circle is needed. Such a force is called a "centripetal" force. Someone in circular motion is therefore constantly accelerated toward the center of the circle. Imagine a person on a merry-go-round, standing on a sheet of graph paper that defines her "frame of reference." Because that sheet of graph paper is moving in a circle, it is accelerated, and is therefore a "non-inertial frame of reference."

   Objects viewed from that frame of reference therefore do not appear to obey Newton's Second Law of Motion. If a ball is thrown by the person on the merry-go-round, its horizontal motion (we are ignoring the pull of gravity and the ball's vertical motion) will not be in a straight line even while it is in free flight.

   But just what will it do? And how can we demonstrate it?

Before you do any calculations, you might try an experiment: Get on a merry-go-round that is spinning fairly briskly (a small one with a short radius spinning fast is best) and try to toss a basketball to a person directly across from you. It is almost impossible to do! If you don't believe that, try it.

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