- #1
greypilgrim
- 513
- 36
Hi.
Assume a rock lying on the ground. Furthermore, let's assume both the rock and Earth to be rigid bodies. I'm a bit confused about force equilibrium and Newton's 3rd law here. There's Earth's gravity acting on the rock, and by Newton's 3rd law (or just by looking at the law of gravity) the rock's gravity acts on earth. But on the other hand, since the rock is at rest, the gravitational force pulling it towards the c.o.m. of Earth needs to be balanced by a normal force of the ground acting on the rock (which is probably due to electrostatic interactions between the two objects).
So what is the reactio to this normal force of the ground on the rock? Isn't it the rock pushing on the ground because of gravity? But then this force would have two opposing forces, first the attraction of the c.o.m. of the Earth and second the normal force of the ground. I guess there's a flaw in my train of thought, but where?
Thanks
Assume a rock lying on the ground. Furthermore, let's assume both the rock and Earth to be rigid bodies. I'm a bit confused about force equilibrium and Newton's 3rd law here. There's Earth's gravity acting on the rock, and by Newton's 3rd law (or just by looking at the law of gravity) the rock's gravity acts on earth. But on the other hand, since the rock is at rest, the gravitational force pulling it towards the c.o.m. of Earth needs to be balanced by a normal force of the ground acting on the rock (which is probably due to electrostatic interactions between the two objects).
So what is the reactio to this normal force of the ground on the rock? Isn't it the rock pushing on the ground because of gravity? But then this force would have two opposing forces, first the attraction of the c.o.m. of the Earth and second the normal force of the ground. I guess there's a flaw in my train of thought, but where?
Thanks