Rajat is sitting in an open cart that moves horizontally with an acceleration of 5 m/s2 rightwards. Rajat is unaware of his acceleration, and throws a ball up in the air. He attempts at drawing a Free Body Diagram of the ball assuming his frame to be inertial. Which of these would closely resemble his free body diagram of the ball (neglect air drag)?
So Rajat saw that the ball is falling down with g as the experiment is happening on ground so there will be a gravitational pull of mg downwards. Also with respect to him the ball has an acceleration of 5 m/s2 leftward.
But how can there be acceleration without a force.
Rajat's belief is that he is in an inertial frame (which isn't true) he knows physics. He knows if there is acceleration then there will be force. So he just assumes a force on the ball in the leftward direction.
This force is a pseudo force as its not real. According to him this pseudo force will be responsible for the pseudo (unreal) acceleration of the block.
So F = ma (pseudo force)
F = (m×5)N
Note: Actually in ground (inertial) frame the ball is just falling down under gravity so Free Body Diagram will have only one force.
While in cart (non-inertial) frame ball has a pseudo (unreal) acceleration with respect to cart's frame so a pseudo force which is equal to mass × acceleration of frame and direction opposite to the acceleration of frame will be applied.