A concrete floor is hard and it does not yield. Therefore, man falling on concrete floor is stopped abruptly in a short interval of time and suffers a sudden change in momentum. Therefore, concrete floor exerts a large force and man gets more injury.
On the other hand, when man jumps on ploughed sand, due to depression of sand, the time of impact increases. Therefore, rate of change of momentum decreases and the person experiences less force. Thus, man gets less injury.
