Question

When we jump on a concrete surface the feet are more seriously hurt than while jumping on a sand

Answer 1

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