# B What kind of force acts in the center of the Earth?

1. Apr 4, 2016

### Gabriele Pinna

What kind of force acts in the centre of the earth ? What is the intensity ?

2. Apr 4, 2016

### BvU

There is quite some pressure there and it is pretty hot as well.

3. Apr 4, 2016

### Svein

And, of course, the force from the sun according to Newton's law (and from several other celestial bodies...).

4. Apr 4, 2016

### Gabriele Pinna

What about the gravitational force acts by the earth ?

5. Apr 4, 2016

### Staff: Mentor

If you assume the earth has a spherically symmetric mass distribution, the gravitational force of the earth on an object at the center of the earth would be zero.

6. Apr 4, 2016

### ProfuselyQuarky

Could you say normal force is acting on the center, or no?

7. Apr 4, 2016

### A.T.

Normal to what?

8. Apr 4, 2016

### ProfuselyQuarky

Normal force meaning any force acting on something at a $90^{\circ}$ (right) angle. If we’re talking about a single point in the center of the earth, would that mean that normal force is acting on the point in all directions?

9. Apr 4, 2016

### Staff: Mentor

A "normal" force is usually a force between an object and some supporting surface (and normal to that surface). What did you have in mind?

10. Apr 4, 2016

### ProfuselyQuarky

Normal meaning orthogonal. I didn’t know there had to be a supporting surface.
I had that in mind. I was wondering if that was correct or incorrect. I guess it's incorrect?

11. Apr 4, 2016

### Staff: Mentor

90 degree angle relative to what? Any direction looks the same, there is no normal force because the concept does not even make sense.
And a force from what?

12. Apr 4, 2016

### A.T.

That is pressure in a fluid, not normal force.

13. Apr 4, 2016

### ProfuselyQuarky

That’s what I was asking about. Since the 90 degrees would not be relative to any specific surface, I thought that would mean that the force could be acting on the point in all directions.
Okay, fine. The idea is jargon.
I see . . . thanks for the explanation :)

14. Apr 4, 2016

### BvU

I figured that's what you were really inquiring about -- and teasingly avoided that in my first answer. The Doc made good on that, but perhaps it's good to elaborate a bit more. For $1/r^2$ laws such as Newton's law of gravity ( and following ) and Coulomb's law one can derive Gauss' theorem that popularly says: you only have to take into account what's underneath you -- the contributions from the part of the sphere with a greater distance to the center cancel.

And it makes sense: along a hole through the earth something has to happen with the force from gravity, because at the other end it has the same magnitude but points the opposite way.