# Gravitational pull

1. Feb 21, 2008

### deargodnoargh

This is incredibly ignorant (and worse, the result of a drunken conversation), so I apologize in advance, but:

If you worked out the mass of an average human being, and suspended them in space at the average distance of the Earth to the Sun (but subtracting the existence of the Earth and moon), and made them immobile (i.e. not drifting in any particular direction or what have you) - would they eventually be drawn toward the sun? Or would they drift toward the nearest planet? Or would they stay where they are because the mass is so small relative to everything else?

What elements would be used to deduce the answer, or if the question is based on fundamentally flawed notions, what are they?

Anyone care to indulge a dullard?

2. Feb 21, 2008

### ||spoon||

well the person wouldn't go anywhere because you made him immobile...

If you hadn't the man would be influenced by the planets near him aswell as the sun. I suppose it depends upon where you placed him but he would feel a force from both. Gravity varies in an inverse square relationship with distance, so you would feel less pull fron masses very far away compared to masses closer (unless the close masses were small and the distant mass huge).

3. Feb 21, 2008

### jhicks

The pull of gravity is still there. This can be seen by newton's gravitation formula, $$F = \frac{Gm_{person} m_{sun}}{r^2}$$ where G is a constant and equal to 6.67e-11, and given that:

the sun weighs ~2*10^30 kg
this person weighs 100kg
mean distance from the earth to the sun is around 150 billion meters

We arrive at the conclusion that the force due to gravity of the sun is near 0.6kg*m/s^2, unless I did that wrong. The number seems too high given that at one point in the day the sun's and the earth's forces are opposed and at other times they are constructive and yet we do not feel any difference. Then again, who could claim to feel a 0.006m/s^2 difference in acceleration.

Edit: Oh boy it's late I screwed up all kinds on this question.

Last edited: Feb 21, 2008
4. Feb 21, 2008

### deargodnoargh

very slow but nevertheless there - so if you had just the sun of our solar system, and no other planets or objects, and the object/person at Earth distance; they would definitely start accelerating towards the sun.

so if it is all determined by the size and distance of nearby masses, would you say that one of the planets close by would capture it first, or the sun?

5. Feb 21, 2008

### Poop-Loops

6. Feb 21, 2008

### ||spoon||

won't the acceleration continue to increase as you get closer due to greater force as r decreases?

7. Feb 21, 2008

### Poop-Loops

Yes.

8. Feb 22, 2008

### D H

Staff Emeritus
You did nothing wrong in calculating the gravitational force due to the Sun on a 100 kg mass at a distance of 1 AU.

That 100 kg person will not, however, experience a daily solar-induced 1.2 newton variation in weight. The Earth is also accelerating toward the Sun at 5.9 mm/s2. The Sun does make our weight change, but not by much. At high noon you are 6378 km closer to the Sun than is the center of the Earth, and at midnight, 6378 km further from the Sun. For a 100 kg person, the solar-induced weight variation is 1.1 millinewtons, not 1.2 newtons.

9. Feb 22, 2008

### BobG

The second one loaded too slow for me to check out, but the first game is pretty cool once you get to the multiple objects.

With two objects, it's not hard to figure out which direction an initially immobile object will go. You just need the mass of the planet(s) you're talking about, which you can google. A better feat is to figure out the net gravitational attraction acting on moving objects. If you place a moving object just right and at the right speed, it will orbit the Sun at the same angular rate as the Earth in spite of being closer to the Sun. In other words, you can place it in a Lagrange point. That's the idea used by satellites that monitor the Sun (SOHO and ACE, for example).

I only checked out the first few levels with multiple stationary objects. Do the blue stars eventually start moving?

10. Feb 22, 2008

### jhicks

Yes I see that now. This is why I did not choose to pursue physics , I can't even keep track of simple newtonian mechanics.

11. Feb 22, 2008

### Poop-Loops

The mechanics may be simple, but it's the "keeping track of" part that's hard. Don't worry, even as a 3rd year student I wouldn't be able to just come up with all of this off the top of my head. I'd still have to sit down and think hard about this.