Question

what cause the person to fall

Answer 1

gravity of course :)

Answer 2

Unbalanced forces. For example while he is free falling, the person would be falling as the force of gravity (mg) would be acting downwards and air resistance would be acting upwards. The air resistance is obviously smaller than gravity(otherwise it would be possible to fly upwards without any effort) and hence you accelerate towards the center of earth and fall down.

Answer 3

well there are two things you have to consider one is net torque about the centre of mass and the net force
When you are falling freely it is unbalanced forces (mg greater than air resistance) which makes you fall but when you trip and fall it is because the net torque about the centre of mass is not zero....

Answer 4

GRAVITY!!!!!!!!!!!!!!!!!!!!!!