# When you are falling down, is the earth moving ever so slightly up towards you?

1. Oct 18, 2007

### ywd

When you are falling down, is the earth moving ever so slightly "up" towards you?

Pardon the n00bocity of this question, but i believe gravity is a force that pulls all objects towards each other, so wouldn't it be pulling the earth towards you? Of course it would be a negligible movement.

2. Oct 18, 2007

### malty

Yes you are, when you drop a stick, the stick is attracted to earth so it will pull the earth towards it, but the earth has a much much much larger mass so its pull on the stick will be more noticeable and much stronger, and as far as I know it's not a noobish question. The only really noobish question is the one not asked.

3. Oct 18, 2007

### Staff: Mentor

Absolutely. Viewed from an outside inertial frame, both object and earth accelerate towards each other. Of course the acceleration of the earth is ridiculously small. (Calculate it!)

4. Oct 18, 2007

### Chi Meson

the force on the stick (from the Earth) is the same as the force on the Earth (from the stick). This is according to classical theory of course. The real difference is the acceleration (as DocAl already noted).

5. Oct 18, 2007

### Rahmuss

Not only that; but the the moon isn't really orbint earth exactly. It's orbiting the Sun while it orbits the earth.

Oops, not quite, because the planets don't really orbit exactly around the Sun, they orbit around the center of gravity for our Solar System (which is close to the center of the Sun). So the Sun really orbits the center of gravity for our Solar System.

Oops, not quite, because our Solar System is actually orbting the center of gravity for our Galaxy.

Oops, not quite, because our Galaxy is actually being pulled on by gravity toward the center of gravity for our cluster of galaxies.

Oops, not quite, because our cluster of galaxies is also being pulled toward the center of gravity for the entire universe.

So, really, everything in the entire universe is being attracted to your body when it falls (or even when you're just standing there). Although the force grows very weak very quickly since the mass of our body is very low really. I guess you could use that as a pickup line:

"Hey baby, I've got an attractive force that reaches the ends of the Universe"

Ok, so it's a bad pickup line. :D

6. Oct 19, 2007

### Shooting Star

But we ascribe all the change of potential energy to kinetic energy to the object falling, and none to the earth, even though the earth also moves, and both have the same momentum in opposite directions. Can you figure out why?

7. Oct 19, 2007

### Chi Meson

Is this a question you'd like an answer to? Or a challenge to the OP? I can give you the answer, but we can let other folks stew in it for a while.

8. Oct 19, 2007

### PatPwnt

So what you are saying is that I am the center of the universe?

9. Oct 19, 2007

### Rahmuss

LOL, well, in a sense that's what science says; but that's not what I'm saying there, no. I'm saying that your gravitational force flows throughout the entire universe. So, unless your mass is the most significant mass in the Universe, then you're not the center (of mass) of the Universe. :)

10. Oct 19, 2007

### nesna

Is it because, even though the earth moves towards the object in accordance with gravitation law, its motion across space due to the attraction with the object is so negligible that the effect the object has on earth is completely ignored.

by the way, are you sure both the object and earth have equal momentum in opposite directions. Isn't mass a huge factor that determines the kind of effect they have on each other?

11. Oct 19, 2007

### Staff: Mentor

Both object and earth experience the same force for the same time, thus they both have the same change in momentum (but oppositely directed, of course).

12. Oct 19, 2007

### Shooting Star

Oh, it was meant for the OP, who supposedly is a newbie, to help him learn some more Physics. That's why I gave his quote.

13. Oct 19, 2007

### nesna

If the object and earth experience the same force for the same time, am i right in assuming that the earth's contribution to that force is much more than what the object may contribute to that force?

14. Oct 19, 2007

### Shooting Star

You are saying that the forces are equal. Then what do you mean by "earth's contribution" is more?

15. Oct 19, 2007

### Staff: Mentor

Not quite sure what you mean, since a force is an interaction between two objects. Certainly the mass and gravitational field of the earth is much bigger, but both objects exert the same force on each other.

16. Oct 19, 2007

### PatPwnt

Yes. The earth makes the force as large as it is more than the person falling. If you were an ant, the force would be still relatively large. If the earth was an ant. Then the force, would almost be non existant.

17. Oct 19, 2007

### Rahmuss

nesna - The change in momentum is the same. Mometum is given by the equation:

$$p = mv$$

And so, yes, the vass of the earth is very large; but the velocity at which it moved when the stick hit it was very small. The force is equal and opposite.

18. Oct 20, 2007

### Shooting Star

You are almost there...