Question

When a moving bus stop suddenly the person standing in the bus tends to fall in forward direction

Answer 1

This is called inertia. This is stated in 1st law of motion stating that an object tends to remain in rest or in motion until and unless an external force is applied. In the above case the person in the bus comes in rest when the bus moves with constant velocity. When braked the person in rest want to go forward direction but brakes are external force.
(Note that the person is in rest with respect to the bus.
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Answer 2

due to force on friction between tyre and road and interia........that's why when we on bus and the driver brake we suddenly fall on each other....... THANK YOU FOR YOUR QUESTION