Quick question on energy and gravity

So, on Earth, you cannot really generate any energy with gravity. You are just releasing energy previously put into the system.

For example, a hydroelectric power generator uses water pressure to generate electricity. The water pressure comes from gravity pulling downward on the water. But that's not where the energy comes from, energy was put into that water via the sun when it evaporated, as it falls back down, and gravity pulls it hard enough to spin turbines, all you are doing is using power from the sun, carried to you via water and gravity.

Now, in space I get really confused. Take a planet, now an asteroid gets close to that planet, and it's course is changed as it's affected by gravity.

Now, to change the course of a moving object requires energy. If a asteroid is traveling straight, you must expend energy in order to stop it, or deflect it.

Where does this energy come from? If you had an infinite number of meteors being sucked into the gravity well of a planet, you would need infinite energy in order to change the path of those meteors, no? Either that, or at some point you "wear out" gravity, but as far as we know neither of those options are possible.

Is the only reasonable explanation that every single last bit of matter is connected at an infinite level, to account for the "closed system" type scenario required where a planet can constantly produce energy to change the course of a moving object, without ever exhausting this source of energy? Gravity decreases exponentially with distance, but if you know anything about exponential functions like that, they never reach zero, meaning the gravity well of my phsyical body extends out throughout the entire universe, no? Never ending?

reply to post by James1982


I think the answer would be that a meteor (depending on the size, if the meteor was the size of the earth it would be a whole different story..) is significantly smaller amount of mass then the sun and earth. The earth is constantly being drawn to the sun, and its rotating and existing in space itself creating its gravity, and so an object that approaches it will interact with its gravity field, as it is interacting with the gravity field and after, its not as if the gravity field is matter and is now 'used' or broken or ruined, gravity is like waves in a medium, and since the earth is constantly in the medium and it is moving, the gravity wakes or waves are ever present.

Gravity is a force. To change the direction or speed of an object requires a force, not energy.

VictorVonDoom
Gravity is a force. To change the direction or speed of an object requires a force, not energy.

The energy expended is the force times distance.

Back to the OP: A planet has kinetic energy due it moving around its star and rotation on its axis. When a body approaches the planet and the "slingshot effect" accelerates and/or changes the direction of the body the kinetic energy of the planet is transferred to the body. In other words the planet slows down !! The effect is as noticable as a grain of sand raising the worldwide sea levels when it is dropped into an ocean.

VictorVonDoom
Gravity is a force. To change the direction or speed of an object requires a force, not energy.

So if a car is moving, it doesn't take any energy to slow it down, or change its course? Or speed it back up (since gravity can speed things up as well, if it doesn't fall into the gravity of the planet)

I'm trying to figure out how both of those can be true at the same time, that it takes no energy to change the path of a planet or meteor, but it takes energy to change the path of a car, a person, etc.

yorkshirelad

VictorVonDoom
Gravity is a force. To change the direction or speed of an object requires a force, not energy.

The energy expended is the force times distance.

Back to the OP: A planet has kinetic energy due it moving around its star and rotation on its axis. When a body approaches the planet and the "slingshot effect" accelerates and/or changes the direction of the body the kinetic energy of the planet is transferred to the body. In other words the planet slows down !! The effect is as noticable as a grain of sand raising the worldwide sea levels when it is dropped into an ocean.

That's the best thing I could thing of, but then I thought about a planet that isn't moving, sitting static. It's still going to have the same gravity, correct? The orbit of the earth, or the spinning of the earth, doesn't create the gravity. The mass does. The movement of that mass has an effect on gravity, but doesn't create the gravity itself.

In other words, if the planet stopped spinning gravity doesn't disappear. If we stop orbiting the sun, gravity doesn't disappear. Our movement has an effect how our gravity effects things, but the gravity isn't created by that movement.

So a static planet, is still going to pull in meteors, and it takes energy to stop a meteor from traveling on it's current trajectory and change it to a new one, I just can't fathom where this energy comes from.

reply to post by James1982


Energy (or work for a better term) is defined as the integral of force along path. "Along" is the important part here. This means you only have to put work into a system, when the force acts along the motion direction.

Planetary (circular) orbits don't require energy as the force is always perpendicular to the motion direction.

In general it is a bit more complicated, but energy conservation makes sure that non circular orbits are possible too.

reply to post by yorkshirelad


Force times distance equals work. Energy is work divided by time.

James1982
Now, to change the course of a moving object requires energy.

This is not always true. It does require some interaction with momentum however.

For instance, a charged particle in a magnetic field will continuously "change course" (i.e. spin around magnetic field lines) but will not directly lose energy.

(actually it's more complicated because the change in direction of a charge also results in radiation being emitted which does result in a loss of energy but this is something peculiar to electromagnetism, not a general principle, and if the initial speed is non-relativistic and the magnetic field isn't super strong the radiation loss is negligible).

If a asteroid is traveling straight, you must expend energy in order to stop it, or deflect it.

To stop it yes, to deflect it, not necessarily. The asteroids orbiting around the Sun are all changing their course (in the Newtonian sense) in their normal orbit all the time but the Sun does not expend energy to do so.

James1982

VictorVonDoom
Gravity is a force. To change the direction or speed of an object requires a force, not energy.

So if a car is moving, it doesn't take any energy to slow it down, or change its course? Or speed it back up (since gravity can speed things up as well, if it doesn't fall into the gravity of the planet)

I'm trying to figure out how both of those can be true at the same time, that it takes no energy to change the path of a planet or meteor, but it takes energy to change the path of a car, a person, etc.

Friction is a force. If an object was moving along with no friction or other force acting on it, it will continue to move in the same direction at the same speed.

You can use energy (as in the case of the car or a rocket) to create a force to change the velocity (speed and direction) of an object. But what you really need to accellerate or decellerate an object is force. The force can be gravity, magnetic, friction, centripetal, etc. The longer the distance over which you apply the force, the more work is done. To do the same amount of work over a shorter time requires more energy.

I hope I'm explaining this well.

reply to post by James1982


Well, my friend welcome to the wonderful mystery that is gravity. It acts on all objects with mass, and between objects with mass. There is nothing to explain the change in course of that object, than the mass of the object it is passing. It influences the asteroid by it's mere existence.

Thanks for the posts guys.

I think I was having a brain fart and forgot that when gravity effects something, it's outside the effects of gravity as it were.

In my example of the meteor, if it falls into the gravity well of a planet, it's not fighting against a "universal gravity" if you know what I mean.

Where as for us to manipulate the path of a meteor with rockets, or whatever, we are fighting against whatever gravitational forces are acting upon that meteor, where as a meteor getting effected by the gravity of a planet doesn't have that issue. I mean, I know that you can have more than one gravitational force acting on a body, but there isn't a universal force of gravity outside of objects that a planets gravity has to fight against to effect it, only the gravity of other objects.

I'm probably explaining this terribly but I think I get it now.

Imagine two ice skaters heading towards each other such that they will pass each other within arm's reach. If they were to grab each other's arms as they pass, they would start spinning around their center of mass. If they then let go again, they would continue straight in a new direction, determined by their momentum at the time. The skaters would then have changed each others direction of movement without expending any energy, for their momentum is conserved, and thus their kinetic energy. The argument can be made clearer if it is assumed that the ice skaters' arms are rigid, thus not being able to perform any sort of work.

I hope that analogy might illustrate more intuitively how energy is not always involved when changing the direction of motion of an object.

Things are of course more complex in the real world. For example, if our two ice skaters were asteroids in orbit around the sun, they might perform a 'twirl' such that one asteroid is hurled into the sun while the other is launched into a higher orbit. In this process, both asteroids would convert some of their potential energy into kinetic energy, and vice versa. In the end, energy is always conserved however.

The action at hand is governed by gravity, which is a central force, and all central forces are conservative forces, named as such because energy is always conserved by such a force.

Deran
Imagine two ice skaters heading towards each other such that they will pass each other within arm's reach. If they were to grab each other's arms as they pass, they would start spinning around their center of mass. If they then let go again, they would continue straight in a new direction, determined by their momentum at the time. The skaters would then have changed each others direction of movement without expending any energy, for their momentum is conserved, and thus their kinetic energy. The argument can be made clearer if it is assumed that the ice skaters' arms are rigid, thus not being able to perform any sort of work.

I hope that analogy might illustrate more intuitively how energy is not always involved when changing the direction of motion of an object.

Things are of course more complex in the real world. For example, if our two ice skaters were asteroids in orbit around the sun, they might perform a 'twirl' such that one asteroid is hurled into the sun while the other is launched into a higher orbit. In this process, both asteroids would convert some of their potential energy into kinetic energy, and vice versa. In the end, energy is always conserved however.

The action at hand is governed by gravity, which is a central force, and all central forces are conservative forces, named as such because energy is always conserved by such a force.

Laid out very clearly, thank you. I was aware of the concept that as the moon gets farther away the earth is slowing down in rotation, I just fell a bit short when applying that same concept to other things.

Thanks again guys! My idea of the world hasn't been shattered then I guess, although I was kind of hoping it would be

Regarding your gravity-related question, it depends on what you believe gravity actually is. The most common explanation is not that there is an energy source causing the change in the orbiting or nearby body, but that the body is following the closest thing it can to a straight path in curved space-time. So it is the actual topographical landscape, ie space-time, that caused the shifts.

Of course there are other theories, such as gravity being caused by an actual particle, among others. Anyway...Everything has energy, because everything has a molecular structure, and as we know, atoms contain energy in a quantity proportional to its mass. There are different forms of energy, as you probably know, and your example regarding energy being stored in water did not seem correct to me.

Take evaporation for instance. I would think there would be more energy that is released from the water than the other way around. Mainly there is just a change from a liquid to a gas, via evaporation. So aside from the energy stored within the atoms of water, the only other sources of energy I can think of would be thermal or heat energy, and potential and/or kinetic energy from the positioning of the water, which could fall and turn a turbine, as you mentioned. I don't know if I even understood exactly what you meant, and I cannot be certain that I am correct regarding the quantity and forms of energy in something like water, as I tend to overlook obvious things sometimes, lol...But I'm fairly confident. Regarding gravity, I don't think I am wrong at all, although the ideas I mentioned could be expanded on exponentially.

reply to post by James1982


The asteroid permanently changed the kinetic energy of the Earth by some "small" amount like (1 + infinity) / infinity

reply to post by James1982


The course of the Earth is also altered, albeit minutely.

This is the same phenomenon as the Moon's effect on Earth. The Earth used to have 6 hour days. As the Moon slowed the rotation of Earth down, the moon was pushed farther away. The moon used to be much much larger in the night sky when it first formed because of how close it was.

reply to post by James1982

Where does this energy come from?

It comes from the angular momentum of the planet, which is decreased by exactly the same quantity by which the momentum of the captured asteroid, etc., increases.

I just can't fathom where this energy comes from.

Nobody REALLY knows the underlying cause of gravity. If anybody in this thread claims to have the answer, don't believe them. Newton could not explain it, Einstein could not explain it, and some guy on ATS that had one semester of physics can not explain it. He probably thinks he can, but what he will really do is DESCRIBE it. Just as Newton did, and Einstein did better, and now Hawking can do.

If you place a bowling ball on a trampoline, it warps the 2D plane and now every lighter object slides toward the ball. Obvious, because the ball and friends are attracted to the massive pull of the earth.

If you place the earth in empty space, it warps the 3D plane and now every lighter object slides toward the earth. Not so obvious, because then what is behind the next curtain? I can picture space warping like a 3D trampoline, but that's where it stops.

Sadly, we cannot think in 4 dimensions, or gravity and Quantum physics would be as obvious as the bowling ball analogy.