Physics 2.20 - 2.22 - Circular motion and inertial mass

Direction in circular motion

Higher tier only

Speed gives only how fast an object moves. Velocity gives both speed and direction. An object in a circular orbit can therefore keep the same speed while its velocity changes continuously, because the direction of motion changes at every point.

At any instant, the velocity points along the tangent to the circle. Equal-length velocity arrows at different positions can show equal speed, but the arrows point in different directions and so represent different velocities. A change in velocity means that the object is accelerating, even when it is not speeding up or slowing down.

To produce that continually changing direction, there must be a resultant force towards the centre of the circle. This inward resultant is called the centripetal force.

Centripetal force names the role of the resultant force; it is not an additional type of force. Depending on the situation, the inward resultant could be provided by friction on a turning car, tension in a string, or gravity acting on an orbiting object.

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If the inward resultant force suddenly became zero, the object would no longer curve. At the instant of release it would continue in a straight line along the tangent, following Newton's first law. It would not shoot radially outward from the centre.