Question

Why does pseudo-force have to be added to non-inertial frames?

Answer 1

Hint Non-inertial frames of reference are those frames where the frame moves with a net acceleration. To use Newton’s laws of motion correctly, we must take into account the acceleration of the non-inertial frame since it contributes to the forces experienced inside the system.

Complete step by step answer
A pseudo force is an apparent force that acts on all masses which are in a non-inertial frame of reference i.e. an accelerating force. The pseudo-force is not from any physical interaction between the objects in the system, but rather from the acceleration of the non-inertial reference frame itself. As a frame can accelerate in any arbitrary way, so can the pseudo forces.
When using Newton’s laws of motion for this non-inertial frame of reference, we must take into account the acceleration of the non-inertial frame of reference which is done in the form of pseudo force. By including the pseudo-force, we will ensure that we are properly capturing the dynamics of the system through Newton’s law of motion.
For e.g., Pseudo force can be used to explain why a pendulum swings backward from a car from whose ceiling it hangs accelerates forwards. This is because the pseudo force that acts on the pendulum acts in the direction opposite of the acceleration pushing the pendulum in the backward direction.

Note
Pseudo forces only act in non-inertial frames of reference i.e. accelerating systems. On a body moving with constant velocity but no acceleration, there will be no pseudo force acting on it. For e.g. in a bus moving with constant velocity, we can stand comfortably but when the bus accelerates we can fall back due to the pseudo force acting on us.