A person hit harder when he falls on a concrete floor than when he falls on a heap of sand from the same height because On a hard surface he is brought to rest in a very short time, so greater force is called into play, whereas, on a heap of sand, he is brought to rest in a longer time, so lesser force is called into play (or explanation in terms of momentum) I cant understand why On a hard surface he is brought to rest in a very short time whereas, on a heap of sand, he is brought to rest in a longer time,
it is because of the particles of matter, in the first case, concrete is rigid therefore it directly opposes the motion of the body but in the second place, sand is a non-rigid surface therefore whenever an object hits it the particles of matter seperate or slip on each other causing the energy to be absorbed and not opposed back. And if u find this answer useful you are welcome.
When you fall on sand, it takes some time for you to come to rest as the sand particles move. But when you fall on hard floor, your momentum becomes zero suddenly. So the change in momentum takes more time in sand than in hard floor. And we know that force acting is equal to rate of change of momentum, which is less in sand than hard floor, the force acting is less in sands than hard floor.