mrzwink asked in Science & MathematicsPhysics · 9 years ago

# i dont get it could someone please explain: Energy from gravity...?

i have been reading up on relativity, and i read that gravity is not a force. we perceive it as a force because we relate all motion to gravity (newton) and gravity is merely a in spacetime.

an object moving in a straight line in spacetime would seem to rotate in space. this makes sense to me, an object caught in a gravity well of a planet would not require energy to change course, as it doesnt change course...

however then i read stuff like this on the wiki of europa:

"This hypothesis proposes that heat energy from tidal flexing causes the ocean to remain liquid and drives geological activity similar to plate tectonics."

if gravity has no energy by simulate the presence of a force how could a gravitational pull of jupiter heat an ocean? where does that energy come from?

could someone explain?

Update:

the missing word is "curvature"

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• rowlfe
Lv 7
9 years ago
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You can understand what apparently is happening on Europa if you just look at it like the Earth Moon system. The moon has an orbital period equal to it's rotation about its axis. THAT is why the same "face" stays facing us. Eventually, the Earth will stop just as the moon has already. Eventually, the Earth and Moon will have the same face, facing the other. The Moon, being only about 1/4 the size of the Earth, simply stopped first. What is happening is gravity causes the tidal bulge on the Earth. The Earth rotates faster then the moon orbits, so the bulge moves ahead of the Moon. The additional mass of the bulge off center ahead of the moon "tugs" on the moon, to drag it along, and at the same time, slowing the Earth's rotation just a little bit. This increases the orbital velocity of the moon, and it moves further away from the Earth. At the same time, the rotation of the earth slowed by a few microseconds.. At present, the moon is receding at the rate of about 1 inch per year as energy is lost by the earth, to the moon. And as a result, the Earth slows down by a few microseconds. Like water seeking its own level, energy that leaves the earth is received by the moon. On Europa, the same effect is happening. Remember one important thing, the earth appears solid, but it really is just a bunch of rocks packed together like a snowball, and exactly like a snowball, can be reshaped by outside forces. It is obvious about the tidal bulge of the oceans, but the same is happening to the crust as well, just on a much smaller scale than the oceans. The earth has a liquid core, so while gravity tends to hold it in a spherical shape, it CAN flex under the moon from gravity, and it DOES. It is thought that this flexing by the moon is one reason why the core is still liquid instead of solid as is the core of Mars. Flexing the crust makes heat from friction. Europa is a more extreme case, being so small compared to Jupiter, that it flexes a lot in comparison, which means even greater heating from friction. We have the sun adding a lot of heat to the earth, but Europa is so far away, the sun is only a minor contributor to the heat input. Our moon likely resulted for a Mars size body hitting the earth with a glancing blow, which is probably where our rotation came from. The moon probably formed from the debris thrown around by the collision, at a distance of about 10,000 miles. The rotation of the earth was at that time about 10 hours. Tidal forces would have been huge. Now, billions of years later, the moon is out at 250,000 miles, and the earth rotates every 23 hours, 59 minutes and change. If it was exactly 24 hours, we wouldn't need leap years. Anyway, THAT is the way I understand why it is possible there is liquid water on Europa.

• 9 years ago

You opened with saying that you're familiar with relativity, which is more than I can say. So I assume that you know that gravity is the mutual attraction of all objects with mass. However I feel that you are wrong on a few points, and that if I can clear these things up for you, everything will make more sense. If I'm wrong, then I should not have answered this, but at least hear me out, and if you could, let me know what you think about what I've said.

I understand the relation of spacetime and space - spacetime is made up of four dimensions. But wouldn't an object moving in a straight line in spacetime stand still in space? I don't think it would rotate in space unless it acted like a top in spacetime - simultaneously rotating and moving in a straight line.

What you said about an object in the gravity well of a planet - I think you're wrong there in two ways. First, unless the object in question is in a perfectly circular orbit (in which the cross product between direction of travel and gravitational force is always zero), energy (potential gravitational energy) IS brought into play. And in that case, something in a circular orbit would have to have always been there, or it would have had to be acted on by some force to be put there. Second, the object most definitely DOES change course, unless there is something here I'm not getting. Gravity affects everything with mass; that idea of a planet changing an object's course of travel is how the Voyager spacecrafts got their speed boosts in the outer solar system.

For your last question about Europa, consider a spring in a realistic environment. If I pull it - say 5 cm - past its natural equilibrium length and then let go, it will retract back to its equilibrium position... and past it. In fact, it will condense back in to almost 5 cm inside its equilibrium before re-expanding. ALMOST. In a frictionless environment, its oscillations would represent an unchanging sine wave, with an amplitude of 5 cm. However, in the real world, friction causes this amplitude to decay over time, and total energy of the spring with it. So you might say, "Hey, where does the energy go?" Well, it goes to friction - heat energy, specifically.

Much of the same thing happens on Europa - all of the parts of the moon's crust collide with each other, producing heat energy through friction, like the atoms in the spring do. Gravitational potential energy becomes kinetic (or elastic) energy becomes heat energy. This is why all moons and all planets are constantly getting smaller and smaller orbits, as the gravitational potential energy (which is related to radius of orbit and orbit period) becomes heat.

Finally, gravity itself is a force and a way to store energy, as it is a conservative force (look it up, it is). Surely you have felt the force of gravity?

Once again, I realize that you have read more than me in this area. So please, please tell me if I'm wrong. By the way, who was the author of the book you were reading? Also, I think you left out a word - there seems to be a speech gap between "merely a" and "in spacetime" in your first paragraph. It just so happens that that gap might be just what would cause all this to make sense for me. :) Thanks, though. Good question.

SIDE NOTE: What Rowlfe said about leap years is wrong; the year is determined by Earth's rotation around the SUN, not the Earth's rotation about its AXIS.

• Anonymous
9 years ago

The ocean follows a slightly different path through spacetime than the body proper does. The surface of the body can only push up, it cannot keep liquid from squeezing transverse.

Similarly, the body proper is not a rigid object, and bits near Jupiter want to go one way, and bits away form Jupiter want to go another.

Friction results in either case.

The energy comes from:

- slowing down Jupiter's rotation

- slowing down Europa's rotation with respect to its orbit (tidal locking),

- transferring angular momentum to / from Jupiter from / to the Jupiter / Europa system,

- some combination of all of these.

• 4 years ago

no person quite is familiar with what gravity quite is yet we do comprehend it pulls thig to the middle of the earth you will desire to check and understand : velocity has the two length( the fee) and course acceleration is the substitute in velocity and ther eis acceleration if a physique is going in a circle or curve despite if it keeps an identical velocity ability can are available in many kinds, action : Kenitic ability = a million/2 m V^2 and potential ability of top mgh and that they are able to interchange in case you ignored too lots ( or did no longer pay any interest) there is not any thank you to verify a years paintings in a single day air has no longer something to do with it F = mA m is mass no longer weight, F a tension is a vector with length and course and A is a vector with course. the path f F is often the path of A yet V could be in someother course designers of curler coasters would desire to truly understand those issues or human beings gets killed. e-m with particular Q. friction is a tension that acts opposide to the path of action

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• 9 years ago

The energy from opisite pulls can cause a reaction in wich heat up the ocean if that's what you were looking for

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