# Old onefallin through the earth

1. Feb 12, 2004

### JoseCapablanca

if there was a hole straight through the earth, top to bottom, and you jumped in, would come flying out the other side upside down, and at which moment would you realize HEY im not fallin down anymore im shootin up.....and no Dante didnt make any sense when he did it

2. Feb 12, 2004

### HallsofIvy

You would feel a decreasing force pulling you down until you reached the center of the earth. It would then reverse and you would feel a force pulling you toward your head (slowing your "descent"). I don't know that you would actually "feel" the reversal. The force would drop to 0 then increase the other way so there wouldn't be any sudden change.

By the way, you would not "come flying out the other side". By conservation of energy, you would stop just as you reached the other side and start back down again. (That's neglecting air resistance. Including it, you would not quite reach the end of the hole, fall "back" down, again not quite reaching the same distance from the center of the earth until eventually, you came to rest at the center of the earth.)

3. Feb 12, 2004

### JoseCapablanca

In my late night ignorance I forgot to mention to neglect gravity and now that you mentioned it, air resistance, picture it happening in a pure vaccum with no gravity involved.

4. Feb 12, 2004

### chroot

Staff Emeritus
In a vacuum with no gravity involved? What's the point of having the Earth there if there's no gravity involved?

If you walked over the hole, you'd just float there. Nothing would happen, because there would be no force.

If you somehow accelerate yourself down through the hole, you would continue with the exact same speed, and in the same direction, forever -- barring collisions with the walls of the hole. You'd just fly off into outer space at the same velocity, forever. That's Newton's first law of motion.

- Warren

5. Feb 12, 2004

### JoseCapablanca

ya but when would i get the realiziaztion that i was no longer going down but up ,it makes no damn sense (the thought not me) example
if i remember right during Dantes DIvine COmedy he made it to the center of the earth and had to turn around to keep going to get to the other side, confussed then , confussed now.

6. Feb 12, 2004

### chroot

Staff Emeritus
Forget Dante.

In the absence of gravity, there really is no good way to determine "up" versus "down." Up is what you want it to be, and down is opposite.

- Warren

7. Feb 12, 2004

### JoseCapablanca

8. Feb 12, 2004

### Cliff_J

How could that work? You'd still have this giant mass and even without the matter directly in front of you, gravity would be pulling on you from all the surrounding mass. The net vector would be slightly reduced, but still pulling you down to the center.

Take your math to 10 decimal places and you'd need to factor in the moon, sun....

Cliff

9. Feb 12, 2004

### HallsofIvy

Let's just note that Dante was not well versed on Newtonian mechanics!

10. Feb 12, 2004

### JoseCapablanca

hahah i take it no one has a good answer ;)
good point on Dante

11. Feb 12, 2004

### chroot

Staff Emeritus
Didn't Jose say "in the absence of gravity?"

- Warren

12. Feb 12, 2004

### chroot

Staff Emeritus

- Warren

13. Feb 12, 2004

### Cliff_J

14. Feb 13, 2004

### HallsofIvy

Has it occured to you that may be because you do not have a good question? You are asking about falling "up or down" but insist that it be done "in the absense of gravity". Unitl you can explain to us exactly what you mean by "falling" or "up or down" in the absense of gravity, I think you have gotten the best answers that can be given!

15. Feb 13, 2004

### FUNKER

would earths gravity be greastest just outside the centre of the earth?

16. Feb 13, 2004

### Integral

Staff Emeritus
The force of gravity reduces steadly all the way to the center, there is no sudden change.

17. Feb 13, 2004

### JoseCapablanca

is it so hard to turn your mind away from the norm and think abstractly. DO I have to say ..ok in "my world" theres no gravity but you have a rocket pack attached to your back and it goes off shootin you down ..is that better sheesh

18. Feb 13, 2004

### Integral

Staff Emeritus
No gravity, remember? No standing, standing implies gravity.

In a universe with no gravity, what is down?

If your rocket has no thrust what happens?

If turtles could fly would snakes walk?

Once again if you want to create a meaningful thought experiment you need to make meaningful assumptions. Your question has been answered, have you not read any of the thread?

19. Feb 13, 2004

### JoseCapablanca

ya I have read the thread and have not goten the answer to the question just well you cant ask that cuz this or that makes that or this impposible....i dont see what the big deal is realy its a simple question

20. Feb 13, 2004

### chroot

Staff Emeritus
In the situation where there is no gravity, and no air resistance -- in other words, no forces -- the only law of motion that applies is Newton's first law:

Objects move with constant velocity in the absence of forces.

If you begin moving in some direction with some speed -- say, by firing your rocket pack, or whatever -- you'll continue moving in that same direction, and at that same speed, forever.