Endercreeper01

No, nothing can be "accelerated to the speed of light".

why not? With gravity, everything accelerates towards another mass at the same rate, regardless of its mass, so even if it gains more and more mass, then it still accelerates towards it at the same rate (actually it accelerates at a faster rate), and eventually it can get to the speed of light

Is it possible to use gravity to accelerate something to the speed

of light?

past the swartzchild radius then could something accelerate to c?

 

This assumes you require energy to generate a force.

 

That is not the case. Forces and energy are different, independent physical quantities. You need to get this very clear before proceeding to study types

you need energy to make a force do work on an object

 

Then work is done on or by some agent.

 

 

You have asked a very reasonable question that often puzzles folks and received a relatively short answer from swansont, as he has many calls on his time here.

 

I have tried to fill in some of the gaps but you don't seem to like responding to more than a small part of posts.

I do not wish to indulge in verbal fencing about this subject.

 

I gave you the answer in post#13.

 

The gravitational energy is inherent in the whole system by virtue of the masses and separations of the particles involved, according to the equations being bandied about. It manifests itself as the (gravitational) potential energy of the system.

 

If either the separation or mass or both change then work is done, perhaps on one or more of the masses or perhaps on an outside body.

 

The work done exactly equals the change in gravitational potential energy.

In other words that is the available energy from the change.

 

In order for the system to have arrived at its present state energy must have been input in the past in the form of work by some agent. We do not necessarily know the details of this, only the quantities.

 

So that is where the energy came from.

 

We observe the effects as 'the force of gravity' in Newtonian mechanics, or alternatively the acceleration due to gravity, but as swansont has already noted, we do not know how this force is generated. That is one of the big question curently being attempted by modern physics.

oh

And I did reply to your posts

 

This assumes you require energy to generate a force.

 

That is not the case. Forces and energy are different, independent physical quantities. You need to get this very clear before proceeding to study types of energy.

 

A simple example is a brick sitting on a table. The table exerts a force on the brick. No energy is involved.

 

Energy is only involved, as uncool said, when something changes.

but the distance between them is changing

 

This assumes you require energy to generate a force.

 

That is not the case. Forces and energy are different, independent physical quantities. You need to get this very clear before proceeding to study types of energy.

 

A simple example is a brick sitting on a table. The table exerts a force on the brick. No energy is involved.

 

Energy is only involved, as uncool said, when something changes.

but the distance between them is changing

 

 

Let's just say I don't necessarily agree. It is an interesting problem and I would need to see more detail. For example, if you are talking about orbiting at 0.86c then, as noted above, you cannot simply use length contraction from special relativity.

actually, this is correct because when you have something orbiting something else with a velocity v, then it is just like with regular length contraction. The length l'=lγ, so if first it blocked 10% and then v=.86c, then γ=1/2, so it is 1/2 the original length and so only 5% is blocked. But the thing with this is that it depends on the relative velocity and so it is relative. Relative to the piece of paper, then the light source is shrinking by that amount, so it blocks 1/2 of the light it did before. To a stationary observer and the light source, then the paper is shrinking and blocking only 5% of the light.

Actually, when the masses are closer together, there is less potential energy. And no, it doesn't violate conservation of energy - the lost potential energy is made up for in kinetic energy.

=Uncool-

gravitational potential energy u=GMm/r, so the closer they are, then the more potential energy

 

As I said, we don't know why the attraction is there.

But does this violate conservation of energy? Because just by these 2 masses going closer together is making there be more potential energy. Energy is being created, and energy cannot be created or destroyed. So does this violate conservation of energy?

In Newtonian physics, the energy comes from the gravitational force and the fact that forces can do work when there is displacement. And since you are bound to ask, that force is present because masses interact with each other. We don't know why that is, but we recognize that it happens and study the effects.

 

 

 

The choice of zero potential energy is arbitrary, since we are typically interested in the changes in potential energy.

but what gives mass the energy to attract each other?

Where does the energy for gravity come from? I know there is gravitational potential energy, but where is it from?

I don't fully understand what you are saying however, yes, the circumference (as measured by an observer on the disk) is 2πrγ.

 

I'm not sure what you mean by "become a partial disk". The disk is still a complete disk, but the geometry is no longer Euclidean.

i mean that because the circumference changes and the radius does not, this means that the angle θ changes. With a full disk, θ=2π, so the circumfrence, which is the same as the arc length of a full circle, is 2πrγ, but then if r does not change, then θ changes. For a full disk, when v=0, then θ=2π, but when v does not equal 0, then θ=2πγ. This means that part of it gets cut off and the angle it makes is 2πγ.

But we are also forgetting that it depends on relative velocities. What are relative velocities like with angular motion?

 

Quite the reverse. As that Wikipedia page says, "For physically reasonable materials, during the spin-up phase a real disk expands radially due to centrifugal forces."

 

But note that the relationship between the radius and the circumference will no longer be the Euclidean 2π (due to the curvature of spacetime that swansont mentioned).

when you have a part of a disk with an angle θ that it makes (with a full disk θ=2π), then it would be how much the arc length s would change because the direction is rotational and since the radius would stay the same and s=θr then it also means θ would be changed so there would be θ'=θγ. With a full disk, then it is just like with the partial disk. In this case, them θ=2π, so then mabey it is just that it would become a partial disk and it would be 2π changing instead of r so then that would make it a part of a disk. Is this correct?

This is the "Ehrenfest Paradox" - note that, like all so-called paradoxes in physics, it only appears to be a paradox. A full analysis is pretty complicated and requires taking the properties of real materials into account as well as general relativity.

http://en.wikipedia.org/wiki/Ehrenfest_paradox

If the circumference contracts, shouldn't that mean the radius should also? Because then if the circumference shrinks, the radius has to.

You know how in Special Relativity then there are things that happen to objects as they approach the speed of light? What would the effects be like when something has angular motion? For example, when something has a velocity close to the speed of light, c, then it's length contracts in the direction of motion. The length l'=l/γ (where γ=(1-v^2/c^2)^(-1/2)). I think it would just be how much it decreases in radius, but is that correct? And also with length contraction, you use the velocity of it relative to something else. The relative velocity that 1 object is relative to another v'=(v1+v2)/(1+(v1v2/c^2)), But what are relative velocities like with angular motion?

But then what would the acceleration be of an object in a gravitational field in general relativity?

I was just wondering in general relativity how you would be able to calculate how strong gravity is? I know that you can calculate spacetime curvature, but how do you calculate the force of gravity in general relativity?