In this situation when using newton's third law, the floor is very rigid and thus when we fall the floor applies
the same amount of force on us with which we were falling on it. The rigidness of the concrete floor makes it difficult for it to absorb some of our force. On the other hand sand is not rigid on impact sine it changes its shape while it absorbs a
large amount of our falling force and a small fraction of this
force is applied on is. That is why we don't get hurt
