# Normal force in space

1. Mar 25, 2014

### shreder

imagine there is a ball on a table on a spaceship that is not moving there is no gravitational force acting on the ball
will there be a normal force

2. Mar 25, 2014

### nasu

Is this a question? Some punctuation will not hurt.

3. Mar 25, 2014

### Staff: Mentor

Not moving with respect to what? If there's nothing pushing the objects together, why would there be a normal force?

4. Mar 25, 2014

### danjordan

Is the table not moving with respect to the spaceship or is the spaceship "not moving"?
In any case, assuming a spaceship lost in space far from any planet or star and detecting practically no external gravitational field, if the objects are in contact you could say that there is a normal force (assuming no friction, then) because each object is attracted to the other gravitationally and that force has to be balanced with another force, the normal.
Else, the ball might go through the table and probably be pulled back and oscillate, or something similar.

5. Mar 25, 2014

### Cosmobrain

First of all, if there is no gravity, how can the ball stay on the table?
Either way, no. There is no normal force

cb

6. Mar 25, 2014

### danjordan

But there's still gravitational attaction between the objects, right? Wouldn't they eventually come into contact?

7. Mar 25, 2014

### SteamKing

Staff Emeritus
Perhaps. Perhaps not. The spaceship has mass, presumably more mass than either the table or the ball.

8. Mar 25, 2014

### Cosmobrain

Oh, whoops. OP said the ball and the table are in a space ship. Well...

I assume the table and the ball are inside the spaceship and the spaceship is totally closed. The force of gravity pulling the table due to the greater mass of the ship will pull everything down (edit: I know there is no up and down in space, but come on...). However, the "ceiling" of the ship will pull it up, canceling it out. However, if we assume that the base of the ship has much more mass than the top, it will attract the table and the ball and there will be a normal force that keeps the ball from crossing the table. Correct me if I'm wrong

cb

9. Mar 25, 2014

### dipole

Normal forces have nothing to do with gravity specifically, they are simply a way to account for the fact that you can't push ordinary objects through one another. If the ball is at rest, touching the surface, and is not experiencing any acceleration, then there is no normal force.

You are correct that the ball and table will attract one another, but this force of attraction will be extremely small.

10. Mar 25, 2014

### Staff: Mentor

If we're going to consider the gravitational forces between spaceship, table, and ball, I want to simplify the problem by choosing the spaceship to be a uniform spherical shell. Now it produces no gravitational force inside, so we only have to consider the gravitational attraction between table and ball. They will be weakly drawn towards one another, and if they finally come into contact the normal force will be equal to the gravitational force between them.

With plausible assumptions about the mass of the ball, the mass of the table, and the shape of the table that force comes out to be something like $10^{-11}$ Newtons (and I won't argue with a power of ten or so in either direction from there) - totally completely negligible, many order of magnitude smaller than the force of random air currents acting on both ball and table.

11. Mar 25, 2014

### jbriggs444

Right or wrong, we're off on a tangent here.

12. Mar 25, 2014

### Cosmobrain

The question then doesn't make sense. There is no way the ball won't feel any force. But hypothetically if it really doesn't, then it seems kinda obvious that there is no normal force.

cb

13. Mar 26, 2014

### shreder

when i said there is no gravitational force acting on the ball i meant no external force to the system

there is simply a ball on a table in space
both objects are at rest
there are no forces acting on them except for the gravitational force between them
(forget the space ship)

i apologise for not being clearer in my original statement

14. Mar 26, 2014

### jbriggs444

I think we still need some additional clarity.

Both objects start at rest or both objects stay at rest?

A literal reading of "no forces other than gravitational" would rule out any contact force between ball and table. That would make the question a bit pointless. Is it what you really mean?

15. Mar 26, 2014

### sophiecentaur

There will be a minuscule 'normal force' due to gravitational attraction. But you already more or less said that. Is there any more to your question than that?

16. Mar 27, 2014

### shreder

all i wanted to know was if without weight there still would be a normal force
this all started because my physics teacher that has only a degree in chemistry thinks she is smart "the normal force has nothing to do with gravity"

17. Mar 27, 2014

### Staff: Mentor

Those are two different things. For example, if the spaceship and the table in it were accelerating at 1g, a scale under the ball would read the same as the weight of the ball on earth and there would be a normal force acting on the ball.... But none of this would have anything to do gravity.

It's true that on earth the easiest way of way of generating a normal force is by letting gravity act on an object resting on some surface. But it's not the only way, and although your teacher may be an annoying chemist, he/she is not clearly wrong here.

18. Mar 27, 2014

### sophiecentaur

Until you know a lot more than your teacher, you should take seriously what she says.
Also, from the words you use, I think you are not as respectful as you should be.
She is correct about the fact that there will be electric forces between the molecules in contact between ball and table. These forces will be much greater than the gravitational ones.
In the vacuum of space, these forces can cause substances to weld together so they are difficult to separate. The effect was a surprise for early Space Engineers.

19. Mar 27, 2014

### Travis_King

If we are assuming that there are zero gravitational affects on the ball and table besides their own gravitational attraction (either they're reaaaallly deep into space, or they're in free fall / orbit), if you were to place the ball on the table and they then both remain at rest, there will be no measurable "normal force". However, if you were to place your right hand on the ball, and your left hand beneath it, under the table, and squeeze, you'll create a normal force.

She's right, the normal force is not restricted to gravitational situations, it's just that that's what tends to happen hear on earth in basic situations.

Think of brake pads on a rotor. When the pedal is depressed and the pads squeeze, there is little to no force produced by gravity, it's all hydraulic pressure causing the forces, yet there will be a normal force on the surface of the rotor.

Last edited: Mar 27, 2014
20. Mar 27, 2014

### Travis_King

Though...the electric forces wouldn't act on the CoM of the ball, so there wouldn't be a "normal force", no? Perhaps momentarily as the two objects fused together.

OP, as far as this is concerned though, I imagine we're assuming that the table and the ball are both non-metallic or that they were brought up to space from our atmosphere, where they've already got a nice protective oxidation layer.