What are pseudo forces?

Dear Vineet,

When several concurrent forces are acting on a body whose resultant is not zero, if we imagine a force to balance the resultant of all the forces, it is called a pseudo force. In normal applications of statics, this concept is not used. But in some cases where the concurrent forces keep changing in direction resulting in the resultant also changing direction, pseudo force concept is used.
For example, if we consider a body of mass m lying on the surface of the earth, then actually only two forces are acting on it: 1) its weight, mg, towards the center of the earth and 2) normal reactive force from the surface of the earth. In simple mechanics, we take the normal force to be mg and say that both these are balancing each other. But strictly speaking it is incorrect as the body must be stationary in the universe if the forces balance. Actually, the body is moving on a circle of radius equal to the radius of the earth along-with the rotation of the earth with constant angular speed. It means that a radial force of constant magnitude and varying direction must be acting on the body which is equal to mv^2/R and whose constantly changing direction is towards the center of the earth. If we were to consider an imaginary static equilibrium of the body, we will have to imagine a pseudo centrifugal force equal in magnitude to mv^2/R acting on the body in a direction radially outwards (away from the center of the earth). Such an imaginary force is called a pseudo force.

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