Introduction to Circular Motion, Velocity Vector

The Velocity Vector

The velocity is not constant.

Notice that the velocity of the object is not constant. Its speed is constant. That is, the size of the velocity does not change. However, since the direction of the velocity changes, and since velocity is a vector, and since vectors have size and direction, then a change in the direction of the velocity is spoken of as a change in velocity. If an object in motion changes direction, it changes velocity. This is often a difficult point for students introducing themselves to physics. It is, though, an essential point to understand. An object in circular motion changes velocity, even though it does not change speed.

The change in velocity vector.

This change in velocity is also a vector. A change in a vector quantity is also a vector quantity. We will see that this change in velocity is aimed toward the center of the circle. That, too, may be a bit hard to understand at first.

The object accelerates.

The fact that the velocity changes means that the object undergoes an acceleration, since an acceleration is present whenever there is a change in velocity. Therefore, we say that an object in circular motion undergoes an acceleration. We will see that this acceleration is aimed toward the center of the circle.

There is an unbalanced force on the object.

Since accelerations are tied to unbalanced forces through Newton's laws of motion, we say that an object in circular motion experiences an unbalanced force. We will see that this force is directed toward the center of the circle.

Several vectors are pointing toward the center of the circle.

Looks like we will see that three vectors are aimed toward the center of the circle: a change in velocity vector, an acceleration vector, and a force vector. These three vectors all point in the same direction. Therefore, we say that in circular motion the change in velocity vector, the acceleration vector, and the force vector all point toward the center of the circle.

The meaning of the word 'centripetal'

The word 'centripetal' means 'center seeking.' Since the acceleration for circular motion points toward the center of the circle, it is center seeking, and we call it the centripetal acceleration. Likewise, the force for circular motion is called the centripetal force since it, too, is aimed at the center of the circle. Do not confuse the word 'centripetal' with the word 'centrifugal.' The first is the correct term used to describe the acceleration and force in circular motion. It is not correct to use the term 'centrifugal' in this context. More about that later.