Ground Reaction Force: What Causes It?

[Robin Andrews]

This may seem like a really basic question (I'm teaching lower school science, and it's not my specialism!), but:

What causes the force which balances gravity, which I think is called "ground reaction force?"

All the descriptions I've seen say that the ground "pushes" up on a stationary object, preventing it from falling through the ground. However, this description seems very "active." Are there some kind of elves heaving away underground to keep the Earth from caving in?

A fair amount of searching has not provided any more convincing description. Could someone here please try to enlighten me?

Many thanks in advance,

Robin.

 

[A.T.]

What causes the force which balances gravity, which I think is called "ground reaction force?"

It's the same electromagnetic repulsion, that prevents you from walking through a wall.

However, this description seems very "active."

The terms "action" & "reaction" in the context of contact forces are completely interchangeable.

 

[Robin Andrews]

Thanks A.T. I seem to remember something about electromagnetic repulsion at the atomic level in this connection. My understanding is very vague though - is it that the electrons of the atoms in my foot are repelled by the electrons of the atoms in the floor if I'm standing still? And is it the pressure from my feet that "activates" this repulsive(!) force?

It's quite surprising to me that the answer involves electromagnetism, and may well be to my students. I'm always on the lookout for simple ways to explain things to children which go a little beyond the "elves" model. Please don't be too horrified that I've been let loose on unsuspecting children with my rather dusty knowledge. They are young and I'm revising quickly!

 

[A.T.]

My understanding is very vague though - is it that the electrons of the atoms in my foot are repelled by the electrons of the atoms in the floor if I'm standing still?

That's good enough if you don't want to get into quantum mechanics.

And is it the pressure from my feet that "activates" this repulsive(!) force?

No. Pressure is force per area. It doesn't "activate" the force, it's simply there when the force is there.

 

[russ_watters]

Since this is just for explaining to kids, why not just say that gravity tries to make objects fall and the ground gets in the way? The details of how the force gets applied aren't really relevant.

 

[sophiecentaur]

Since this is just for explaining to kids, why not just say that gravity tries to make objects fall and the ground gets in the way? The details of how the force gets applied aren't really relevant.

You could show them the effect of compressing a soft object** with a mass placed on it - compressed until the repulsive forces balance the weight force. Then progress to the fact that the ground deforms 'a little bit' until it supports your weight. (everything deforms by a finite amount under an impressed force).
** Sponge, rubber mat etc.