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What is pseudo force when it is applied ?

Pseudo force is also called fictitious force.

A fictitious force arises when a frame of reference is accelerating compared to a non-accelerating frame.

For example if you consider a person standing at a bus stop watching an accelerating car, he infers that a force is exerted on the car and it is accelerating. Here there is no problem and the pseudoforce concept is not required

But, if the person inside the accelerating car is looking at the person standing at the bus stop, he finds that the person is accelerating with respect to the car, though no force is acting on it. Here, the concept of pseudo force is required to convert the non-inertial frame of reference to an equivalent inertial frame of reference.

Another example

Consider a ball hung from the roof of a train by means of an inextensible string. If the train is at rest or is moving with a uniform speed in a straight line the string will be vertical. A passenger will infer that the net force acting on the ball is zero.

If the train begins to accelerate, then the string will be making an angle with respect to the vertical. For the passenger, there are only two forces and they are not collinear. But, the ball remains apparently in a state of equilibrium (as long as the acceleration of the train is constant). Here, the concept of pseudoforce is required.

Pseudo force to be applied is equal to the mass of the body under consideration multiplied by the acceleration of the frame of reference and the direction of pseudoforce is opposite to the direction of acceleration of the frame.

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