Can anyone explain physics

Can anyone explain physics?
I understand that a larger force is required to stop another force in a shorter period of time. But say two people of the exact same mass fall at the exact same rate and one hits concrete and another hits grass. When they hit the floor they will exert the exact same force unto the ground, won't they? And the force will exert the exact same force unto them? So why do we stop alot quicker on concrete than grass. And why, according to Fnet=m(v-u)/contact time will less force be exerted on the man landing on the grass? Where does the extra force applied to the man landing on the concrete come from if they land with the same force????
PLEASE HELP :)

Force = Mass x Acceleration =[m(v-u)] / time

You stop quicker on concrete because it physical properties help it resist a much larger compressional forces than grass.

A large force acting for a short time can have the same effect as a small force acting over a greater period of time.

One of the definitions of a force is the rate of change of momentum (change in momentum divided time).

Both cases the change in momentum is the same [m(v-u)] but the time taken to change is different, therefore the force has to change.

Another example; somebody in a car travelling a 60mph, they press the brakes gently and stop in 15 seconds, the force from the seat belt is not very big. Imagine if they slam the brakes on and the stop in 3 seconds; the seat belt cuts into their shoulder. In both cases they have had the same change in momentum, but it has taken different lengths of time, therefore different amounts of force are needed.

Getting back to the question, because the concrete can exert a larger force on the falling man, less time is needed to stop the man.

Why can’t you think of falling on a vertical spring with spring constant k?

First consider that a mass is just placed on the top of the spring.