Question

Why newtons laws of motion do not hold in non-inertial frame and why do we apply pseudo force? Please answer these two questions??

Answer 1

This is because newton's laws are valid only for inertial frames. Analysis from an inertial frame isn't possible using those laws, to put it simple.Let us assume a glass lift is going up with an acceleration 'a'. with respect to the ground. Clearly, this is a non-inertial frame as it has an acceleration. Man 'A' is inside the lift and wants to analyze the motion of the lift, but he cannot, as the newton's laws are not valid. So he takes his phone and calls Man 'B', who stands outside on the ground (in an inertial frame). This fellow watches Man 'A' go up with acceleration 'a'. Forces acting on Man 'A' are:1. Weight, mg2. Normal reaction offered by the lift, N Thus, taking upward direction as positive and using Newton's Law, he writes the equation, N - mg = ma, or N = mg+maHe tells "N=mg+ma" on phone to Man 'A'.Now, Man 'A' looks around himself. According to himself, he is at rest. So, according to him, N = mg. Sadly, he knows this is not a correct analysis, as the newton's laws can't be applied! The correct analysis, as received on phone, is N=mg+ma!