Circular motion can be analysed from two different frames of reference.

Centrifugal Force

Circular motion can be analysed from two different frames of reference. One is the inertial frame (which is either at rest or in uniform motion) where Newtonâ€™s laws are obeyed. The other is the rotating frame of reference which is a non-inertial frame of reference as it is accelerating. When we examine the circular motion from these frames of reference the situations are entirely different. To use Newtonâ€™s first and second laws in the rotational frame of reference, we need to include a pseudo force called â€˜centrifugal forceâ€™. This â€˜centrifugal forceâ€™ appears to act on the object with respect to rotating frames. To understand the concept of centrifugal force, we can take a specific case and discuss as done below.

Consider the case of a whirling motion of a stone tied to a string. Assume that the stone has angular velocity *Ï‰* in the inertial frame (at rest). If the motion of the stone is observed from a frame which is also rotating along with the stone with same angular velocity *Ï‰* then, the stone appears to be at rest. This implies that in addition to the inward centripetal force âˆ’*m**Ï‰*2*r* there must be an equal and opposite force that acts on the stone outward with value + *m**Ï‰*2 *r*. So the total force acting on the stone in a rotating frame is equal to zero (âˆ’*m**Ï‰* 2 *r* + *m**Ï‰*2*r* = 0). This outward force +*m**Ï‰*2*r* is called the centrifugal force. The word â€˜centrifugalâ€™ means â€˜flee from centerâ€™. Note that the â€˜centrifugal forceâ€™ appears to act on the particle, only when we analyse the motion from a rotating frame. With respect to an inertial frame there is only centripetal force which is given by the tension in the string. For this reason centrifugal force is called as a â€˜pseudo forceâ€™. A pseudo force has no origin. It arises due to the non inertial nature of the frame considered. When circular motion problems are solved from a rotating frame of reference, while drawing free body diagram of a particle, the centrifugal force should necessarily be included as shown in the Figure 3.45.

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11th Physics : UNIT 3 : Laws of Motion : Centrifugal Force |

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