Centrifugal and Centripetal Force
Centripetal force is the central force felt by an object moving in a circle. It is the force that is necessary to keep the object moving in this circle, otherwise, the object would follow a straight line path, in accordance with Newton's Laws. Assuming this circle is of radius, r, and the object moves with constant velocity, v, then the centripetal acceleration is:
The centripetal force, Fc, which is directed towards the center of the circle, is then:
Centrifugal force is not a true force at all; it is an effect seen when the observer is assuming a rotating reference frame is an inertial frame. This is done quite often with systems such as the earth, since it is only a factor when the rotation is high in comparison to the size of the system.
To see how this force behaves, consider a two liter soda being carried home in the car. On the way home, you round a sharp left hand curve and the soda bottle slides to the right and crashes into the door. To you, it seems that the bottle has been moved by some invisible force towards the door. Hence we have the fictitious centrifugal force. To an outside observer, the bottle has just obeyed Newton's Law; an object in motion will continue to move in a straight line unless acted upon by an outside force. This is merely an application of inertia. Thus, since you neglected to restrain the soda, it continued to move in a straight line while you turned. It was the force of the door pressing against it that caused it to move, in the end.
Even though centrifugal force isn't a "real" force, it is a real effect and can be applied to our advantage. Consider the Centrifuge apparatus; its use is wide spread. It uses the centrifugal force, which is really an application of inertia, to separate fluids into layers such as cream and milk or plasma and serum. The liquid is a solution made up of particles with different masses. Each particle has an inherent amount of inertia proportional to its mass. As the liquid is spun, the amount of inertia, or the amount of resistance to force, causes the particles to resist being moved in a circle. Thus, more massive particles, being more resistive to movement off their straight line path end up at the bottom of the vessel while less massive particles end up at top.
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