# Basic Gravity Question

1. Apr 5, 2014

### student34

When I throw a ball up vertically in the air, why does it rotate with the Earth around the Earth's axis? What force is acting horizontally above the Earth's surface to keep the ball above the point that I threw it as the Earth rotates as well as orbits the Sun? Does the Earth's rotation cause a gravitational torque or something like that?

So just imagine someone on Mars looks through a telescope and sees me throw the ball up in the air. What forces would the Martian assume is keeping the ball above the point that I threw it?

Last edited: Apr 5, 2014
2. Apr 5, 2014

### Bandersnatch

What does the Newton's first law of motion say?

3. Apr 5, 2014

### UltrafastPED

Ignoring air resistance - you and the ball are co-travelling with the earth's surface prior to tossing the ball upwards.

This co-travelling velocity is tangential to the earth's surface, and is fast enough to make the earth go around once in 24 hours. At the equator this is 40,000 km/24 hours. This very rapid horizontal motion is not noticed by us except on windy days - the tropospheric atmosphere near the earth's surface mostly travels with the ground, but not always.

Now consider the vertical speed of your tossed ball - only a few meters per second. Thus most of the speed is along the surface of the earth, and for the limited time of your toss-up game you will not notice the relative motion of the earth.

But if you throw things really, really high (or drop them from a very high tower) it is possible to note some slippage. Which way will the Martian expect this slippage to occur? Will it be different on the earth with its atmosphere from what would be observed if you were on the airless moon?

Note: no extra forces come into play here. Gravity is always pulling towards the center of the earth (or moon), and everything else is initial velocities, vertical and horizontal.

4. Apr 5, 2014

### student34

I think that I got it. Is it that my particles were added to the Earth at just the right speed such that the ground, ball and I are basically satellites that orbit the Earth at a tangentially mutual speed? And is the reason why we do not sink into the ground or leave Earth's orbit because our acceleration going away from the Earth is equal to Earths gravity g?

5. Apr 5, 2014

### kyrill

There is a general conclusion that objects cannot occupy the same space, even in a black hole, although the 'arrangement' in space is done very efficient;)

6. Apr 5, 2014

### Staff: Mentor

You are not in an orbit, but everything around you moves with (nearly) the same speed, yes.

No. Then everything would float around. You don't sink into the ground because of electromagnetic interactions between the atoms of you and the atoms of the ground.

This is wrong.
Macroscopic objects will repel each other if you try to "combine" them, but in quantum mechanics, multiple objects at the same place are no problem. You can even do the same thing with superfluid helium on a macroscopic level. Two different liquids in the same place, with independent flows.

Last edited: Apr 5, 2014
7. Apr 5, 2014

### student34

But matter among other matter with the same density does kind of "float": water, air, layers of dirt etc.

8. Apr 5, 2014

### Staff: Mentor

This first requires that the matter cannot pass through each other. And you do not have the same density as air.

9. Apr 5, 2014

### kyrill

"This is wrong.
Macroscopic objects will repel each other if you try to "combine" them, but in quantum mechanics, multiple objects at the same place are no problem. You can even do the same thing with suprafluid helium on a macroscopic level. Two different liquids in the same place, with independent flows."
__________________
thx for the correction.
"macro objects" consist of considerable space. So in the macro level objects cannot invade these inner spaces because of as you said "electromagnetic interactions "
What happens to these electromagnetic interactions under conditions with supra helium? Does it allow the second liquid to enter these spaces first forbidden?
In quantum mechanical realms objects are not comparable to macro objects any more and are called particles and not often objects? But can even fundamental particles (with a wave property) in actual reality occupy the same space? If some can, they cannot collide or influence their path?

If particles with mass can occupy the same space, how can a black hole grow?

Last edited: Apr 5, 2014
10. Apr 5, 2014

### Staff: Mentor

In superfluids, the description "there is an atom at this place" becomes meaningless - "passing through each other" is really a macroscopic description.
I don't understand that question.
What is "actual reality"? In our best theories to describe those objects, they can.
They can still interact with each other. Actually, interaction is stronger if they do.
The size of a black hole usually refers to its Schwarzschild radius - the point where gravity reaches a specific strength (simplified description). This has nothing to do with the size of regular objects.

11. Apr 5, 2014

### kyrill

ok thx
but: "he size of a black hole usually refers to its Schwarzschild radius - the point where gravity reaches a specific strength (simplified description). This has nothing to do with the size of regular objects. "
does not convey black holes are smaller than its S. radius, explaining their invisibility. It is not so unlikely to assume that when this Schwarzschild radius grow especially as the black hole (BH) catches or absorbs new mass, it grows.
Does not the growing 'proofs' in the end even particles cannot collapse in each other? If yes they can, how does a BH grow?
I apologize for that question. at that level it is all guessing :)
In my original reply I never intended to include sub atomic realities. Still I am very curious to your mentioned superfluids how they can invade the 'space 'first blocked by EM forces

Last edited: Apr 5, 2014
12. Apr 5, 2014

### Staff: Mentor

13. Apr 5, 2014

### kyrill

hmm
I referred to earth as well, wasn't it you who introduced particles physics in order to prove how wrong my original answer was to the original poster? Then I responded on yr responses in order to understand you. I have no specific interest in black holes.
my apology for the misunderstanding

14. Apr 5, 2014

### ZapperZ

Staff Emeritus
You need to pay closer attention. DaleSpam post to you was his FIRST post in this thread. So it certainly wasn't him who "... introduced particle physics..." in this thread.

In any case, you are hijacking this thread. Please let us answer the OP's question, which did not require us to invoke any the issues that you had brought up.

Zz.

15. Apr 5, 2014

### sophiecentaur

Doesn't that statement contradict the Pauli Exclusion Principle for Fermions? On the face of it, it seems to.

16. Apr 5, 2014

### WannabeNewton

No. The PEP says a composite system of identical fermions must have an antisymmetric state, meaning the fermions making up the (factorized) component states cannot be the same. But the component states "not being the same" means all eigenvalues characterizing the component states cannot be the same-position isn't the only factor.

17. Apr 5, 2014

### yuiop

Hey WBN! When did you last clear out your mail inbox? Tried to send you a pm ;)

18. Apr 5, 2014

### Infinite/Zero

According to question asked by Student34