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I know this is impossible, but humor me, If you had a straight hole that went completely through the Earth and dropped a pebble in would it fall through or would gravity stop it dead straight in the center?

2007-01-27 12:46:20 · 18 answers · asked by the walrus 1 in Science & Mathematics Physics

18 answers

If we ingore the heat of the core (which we'd have to to get a hole through it) the pebble would fall down, accelerating and it's momentum would carry it past the center of the Earth and it would speed up toward the top of the other side of the hole, but decelerating so that it would not emerge from the hole. Then it would drop back down and would occilate back and forth like that through the hole, losing speed from friction and eventually come to rest at the center of the Earth.

2007-01-27 12:49:47 · answer #1 · answered by Anonymous · 3 1

To answer this question, first you have to ask, what causes the earth's gravity? The answer is its its mass - that is the mass of all the dirt, rocks, etc. Now when you go deep enough into the earth, gravity actually starts to get weaker! This is because more and more ground is actually above you! The earth that is above your depth starts to pull you up, canceling the force from the dirt below you. When you are in the very center of the earth, there is an equal amount of "dirt" (or magma, or whatever) around you in every direction. So, all of the gravitational pulls cancel out, and there is absolutely no net gravity in the center of the earth! This is a special case of what is called Gauss's Law.

Any way, all the way down, your pebble would speed up as anything does when it falls, but its acceleration would be constantly decreasing until it reaches the center, where it would keep going, but the balance of gravity would start to pull the pebble back up (towards the center of the earth). The pebble would slow down all the way as it keeps falling away from the center until it stops, then starts to fall back down again, starting the process all over.

The motion would very closely resemble (and is mathematically identical) to the motion of a spring bouncing back and forth, or a pendulum swinging back and forth, what is called Simple Harmonic Motion.

2007-01-27 21:40:16 · answer #2 · answered by Tony O 2 · 0 0

No. It will execute a simple harmonic motion.

For discussion, let's take that the hole starts in north pole and ends in south pole.

Dropping a pebble in the start point in north pole will make it rush to the center. Since except the center point itself, the pebble is always under influence of gravity, it's velocity, thus momentum increases.

But the force decreases with depth, making acceleration varying as negetive displacment.

At the center, momentarily the force of gravity becomes zero, but momentum preserves, and the momentum drives it further towards south pole. This time force of gravity increases as it nears the pole.

At the pole, it's kinetic energy completely becomes gravitational potential energy. For stability, it tries to lower potential energy level. The force of gravity again pulls it down towards the center, and the cycle repeats on.... This is a simple harmonic motion.

2007-01-27 20:59:09 · answer #3 · answered by The madman who makes people fly 2 · 1 0

Harrypete has given a very simple and correct answer, of course neglecting any frictional forces. But your thinking that would it stop it dead in the centre, prompts me to tell you that what drops dead at the centre is the net force due to all th particles of the earth on the pebble. But by that time pebble has already got some velocity which make it pass through tis point to get increasingly attracted towards the centre by all the particles of earth.

2007-01-27 21:08:34 · answer #4 · answered by Let'slearntothink 7 · 0 0

If you evacuated the hole so there was negligible wind resistance, the pebble would actually reach its maximum speed as it passed through the center of the earth. Then it would slow down as it rose to the other end of the hole, coming to rest just as it reached the same distance from the center that it started with originally. Then it would fall back into the hole... and so on and so on.

2007-01-27 20:56:35 · answer #5 · answered by Dennis H 4 · 2 0

The Earth rotates. Your pebble would hit the side of hole, and friction would bring it to a stop before it gets to the center the first time. Without the rotation, the people who are talking about oscillation and eventually coming to rest at the center would be correct.

2007-01-27 20:59:18 · answer #6 · answered by Anonymous · 1 0

i mostly agree with those who say the pebble would fall thru the centr, the momentum would carry it to the China side of the world. but i think that due to wind resistance & air compression occuring in the hole, this would reduce each sequencial pass thru centre. over yrs it would eventually come to rest at center of earth & float there because of all directions of equal & opposite forces of gravity holding it in same place.

but Melvana makes a good point in warning not to do that.

depending on the diameter of pebble in respect to the diameter of hole, movement of pebble thru hole could cause air compression on the China side while also causing air decompression on our side of the world. the resulting compression on the China side would force air blowing up thru their side of the hole, probably flipping up some pretty girl's dress or mini-skirt, & then she would run away from the hole, or step back in surprise from such a phenomenon, & that would result in the aforementioned pebble NOT stubbing her toe.

this air-compression/decompression complimentation would not only reduce each sequential pass of the pebble, but may also be enough to shift air-currents between you & China side. so U may be responsible for Global climate discrepancies between here & China.

a last thing to consider- is the earth's core is usually hotter than the surface. so as the pebble falls thru center, the compressed, surface cooler air would push some of the Center or earth (hotter) air towards the China side & as this hotter air approaches China, it would expand in presence of surface air. on pebble's return trip to US side, it would do the same thing expelling expanding hot air on the US side. this would help in Global cooling, starting at center of earth.

how far have you dug up to now?

2007-01-27 21:31:19 · answer #7 · answered by carbazon 3 · 1 0

No, it would build up a lot of momentum and go past the center...then fall the other way and build up momentum and go past the center the other way, but not as far...

Sooner or later, it would stop in the center.

Your hypothetical would be cooler if it was about hocking a loogie.

2007-01-27 20:49:52 · answer #8 · answered by Anonymous · 2 0

actually, gravity would pull it back and forth around the center of the earth. so it would kinda bounce up and down, not stay in one spot for too long.

2007-01-27 20:51:40 · answer #9 · answered by Macroniencheese 2 · 1 0

i think it would float in the middle due to the poles and gravity

2007-01-27 20:53:48 · answer #10 · answered by undercovernudist 6 · 0 1

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