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thought experiment

X

xyyz

Diamond Member
if we can have massless string and frictionless pulleys, why not the following?

please correct me if i'm wrong, but as far as i understanding goes, terminal velocity is reached when you have friction that acts as a blanace to acceleration.

now assume you have a vaccuum, and a gravity source that is infinitely far away pulling on a ball. this ball would accellerate through space for all of time. now since there is nothing that can generate anything to counter the balanace of gravity, there would be no terminal velocity.

so, would this ball continue to accellerate indefinately, possibly beyond the speed of light? or would some other factor cause the ball to slow down? if there is another factor, what is that factor, and when does it kick in, and how does it kick in?
 
naa, I don't think there would be terminal velocity, the only thing that would cause it to not speed up to the speed of light would be the weight factor and the "fuel/energy" required to bring that to the speed of light. Nothing can go at the speed of light besides light, the energy required to bring even a gram of anything to the speed of light would be enormous. So to answer your question where you have gravity as the pulling force,
1. If its far away enough, the pulling force would be greatly reduced therefore not likely pulling it in the first place. If it was close enough to be pulled in, the time it would take to reach the speed of light would far exceed the time it takes for the gram or atom of what ever to get to the central point of where the "gravity is coming from".
2. Now you say that it's infinitely far away, thing is, what is infinitely far away that you know of? nothing, so right there it makes it impossible for it to occur.

Also, for something to have such a high amount of pulling force and to be infinitely far away and to pull anything, would also be impossible to achieve because in order to have gravity, you need mass. And to get a lot of gravity, you need lots of mass or spinning mass but you still need mass. It's just not possible, the thing limiting the what ever from achieving the speed of light would be the fact it would plow right into what ever was harnessing all this gravity which is the driving force for this "what ever".

Look, if you could have a pulling force that was infinitely far away, then I'm sure what ever your talking about could reach the speed of light but you'd be breaking many laws of physics in order to achieve it. It's just not possible😉
 
Originally posted by: xyyz
if we can have massless string and frictionless pulleys, why not the following?

please correct me if i'm wrong, but as far as i understanding goes, terminal velocity is reached when you have friction that acts as a blanace to acceleration.

now assume you have a vaccuum, and a gravity source that is infinitely far away pulling on a ball. this ball would accellerate through space for all of time. now since there is nothing that can generate anything to counter the balanace of gravity, there would be no terminal velocity.

so, would this ball continue to accellerate indefinately, possibly beyond the speed of light? or would some other factor cause the ball to slow down? if there is another factor, what is that factor, and when does it kick in, and how does it kick in?
Click to expand...



No. The terminal velocity is the speed of light

When matter approach speed of light, their mass increases.

Because of this increase in mass, according to Newton's Second law for Acceleration, a = F/m

Since the gravitational force F is constant, and mass increased, acceleration decreases

And ultimately you end up with 0 acceleration and with a velocity of speed of light.
 
Originally posted by: goku
naa, I don't think there would be terminal velocity, the only thing that would cause it to not speed up to the speed of light would be the weight factor and the "fuel/energy" required to bring that to the speed of light. Nothing can go at the speed of light besides light, the energy required to bring even a gram of anything to the speed of light would be enormous. So to answer your question where you have gravity as the pulling force,
1. If its far away enough, the pulling force would be greatly reduced therefore not likely pulling it in the first place. If it was close enough to be pulled in, the time it would take to reach the speed of light would far exceed the time it takes for the gram or atom of what ever to get to the central point of where the "gravity is coming from".
2. Now you say that it's infinitely far away, thing is, what is infinitely far away that you know of? nothing, so right there it makes it impossible for it to occur.

Also, for something to have such a high amount of pulling force and to be infinitely far away and to pull anything, would also be impossible to achieve because in order to have gravity, you need mass. And to get a lot of gravity, you need lots of mass or spinning mass but you still need mass. It's just not possible, the thing limiting the what ever from achieving the speed of light would be the fact it would plow right into what ever was harnessing all this gravity which is the driving force for this "what ever".

Look, if you could have a pulling force that was infinitely far away, then I'm sure what ever your talking about could reach the speed of light but you'd be breaking many laws of physics in order to achieve it. It's just not possible😉
Click to expand...



While what you said was true, it isn't the main culprit of the problem. Read my post.

Notes to add:

Gravitational attraction is proportional to the product of the two masses and inversely proportional to the SQUARE of the distance


Let's assume the distance is not infinite, but is SUFFICIENT for the mass to accelerate to speed of light and beyond if general relativity does not apply

I think this forms a better situational describtion for the question of which the OP should have asked.
 
Nah, E=MC² clearly says that the mass would be too large to accelerate it past the speed of light. Even in frictionless space. Since you are using an infinitely far away gravity source, there would be no effect on the object.
 
Originally posted by: edro13
Nah, E=MC² clearly says that the mass would be too large to accelerate it past the speed of light. Even in frictionless space. Since you are using an infinitely far away gravity source, there would be no effect on the object.
Click to expand...


lol sorry but that's not it cause E-MC^2 has nothing to do with kinetic energy nor acceleration. It describe the link of the Mass-Energy equivalent
Newton's 2nd law would have been a better explanation, i.e. what I said 3 posts earlier.
 
look... take it at face value.

there are no such things as massless string and frictionless pulleys, but they're constantly used in thought experiments.

so keep the assumption, that gravity is constant, and the distance is infinity.

so far AnnihilatorX has given by far the best explanation given this criteria, so thank you.
 
The velocity would asymptotically approach c but would never reach c itself as the relativistic mass increases with velocity. To accelerate a massive object to c would require infinite energy.
 
Originally posted by: AnnihilatorX
lol sorry but that's not it cause E-MC^2 has nothing to do with kinetic energy nor acceleration. It describe the link of the Mass-Energy equivalent
Newton's 2nd law would have been a better explanation, i.e. what I said 3 posts earlier.
Click to expand...
It depends on what M you're talking about - if M is the relativistic mass then it does include kinetic energy. If M is the rest mass then E=MC^2 is valid only at zero velocity and you have to use the full E^2=p^2*c^2+m0^2*c^4 expression, where m0 is the rest mass. Either way, it's crucial to understanding the problem in the correct special relativistic sense, as Newton's laws are no longer valid.
 
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