Physics Marathon 2012 (1 Viewer)

[bleakarcher]

ah yeh that makes sense.

 

[someth1ng]

If you increase an object to the speed of light, c, then t=0. This causes space-time is be spreaded proportionally and so it works against the formation of a black hole that bunches up the space-time.

Is this correct?

 

[IamBread]

Just a note, this is not v = c, so there is still length and there is still time. And remember, so if you decrease d, you increase the force of gravity at the point, which would just create a 'stronger' black hole.

 

[someth1ng]

This is my new answer:
The mass increase is only observed as it is in a different frame of reference. Theoretically, if you were moving along side this object and shone a flash light on it, it would be able to absorb/reflect that light like it were stationary and its mass would appear to be its rest mass. A black hole is able to act as a black hole (lol) because it has a very large rest mass.

 

[IamBread]

This is my new answer:
The mass increase is only observed as it is in a different frame of reference. Theoretically, if you were moving along side this object and shone a flash light on it, it would be able to absorb/reflect that light like it were stationary and its mass would appear to be its rest mass. A black hole is able to act as a black hole (lol) because it has a very large rest mass.

This is basically the answer. Gravity is only dependent upon the rest mass of the object. The relativistic mass does not contribute to the strength of gravity from the object. And, as you said, if you change your FOR to the FOR of the moving object, then it is now at rest and has no more relativistic mass. If you want a full, proper explanation, you'll have to learn GR

 

[someth1ng]

The relativistic mass does not contribute to the strength of gravity from the object.

After looking around, apparently it does...

 

[IamBread]

After looking around, apparently it does...

Are you sure about this? This is the first time I've heard this. Where did you find this?

 

[someth1ng]

Are you sure about this? This is the first time I've heard this. Where did you find this?

Does Gravity Increase as Mass Increases?
Does a particle's relativistic mass increase result in a corresponding increase in the gravity field generated by the particle?

Yes, the particle has an increase in energy (kinetic and mass increase) and under general relativity, it is energy that generates a gravity field, not just rest mass.

Dr. Eric Christian

Source: http://helios.gsfc.nasa.gov/qa_gp_gr.html#particle

 

[IamBread]

Interesting... it appears that relativistic mass does not contribute to a black hole. http://www.physics.adelaide.edu.au/~dkoks/Faq/Relativity/BlackHoles/black_fast.html

When an object approaches the speed of light, its mass increases without limit, and its length contracts towards zero. Thus its density increases without limit. Sometimes people think that this implies it should form a black hole; and yet, they reason, since its mass and volume haven't changed in its rest frame, it should not form a black hole in that frame—and therefore not in any other frame either. So does a black hole form or not?

The answer is that a black hole does not form.

I think I just assumed that because it doesn't form a black hole, it doesn't contribute to gravity. I think this has come from a misunderstanding of black holes. Yay more to learn .
Here is another page that might be interesting to read.
http://www.physicsforums.com/showthread.php?t=233740

 

[someth1ng]

Yeah, it's definitely outside the HSC syllabus but I think HSC-wise, it's enough to just say that it is because it's an observed mass and not it's "true mass".

PS I'll trust the NASA website over anything else

 

[IamBread]

I think your explanation

The mass increase is only observed as it is in a different frame of reference. Theoretically, if you were moving along side this object and shone a flash light on it, it would be able to absorb/reflect that light like it were stationary and its mass would appear to be its rest mass

is pretty much right for a hsc-level answer.

 

[Parvee]

This thread needs another bump

The escape velocity for planet X is 25000ms^-1. However, a spaceship traveling with its engine on and at a lesser speed, near the planet's surface, can escape the planet's gravitational field.
With clear reference to Newton's concept of escape velocity, explain how this is possible. (3 marks)

 

[SpiralFlex]

This thread needs another bump

The escape velocity for planet X is 25000ms^-1. However, a spaceship traveling with its engine on and at a lesser speed, near the planet's surface, can escape the planet's gravitational field.
With clear reference to Newton's concept of escape velocity, explain how this is possible. (3 marks)

Independent 2011. I just did this today. I do not want to do it again. We had a class discussion about how answers were wrong.

 

[Parvee]

Independent 2011. I just did this today. I do not want to do it again. We had a class discussion about how answers were wrong.

What was the answer given? :O

 

[SpiralFlex]

What was the answer given? :O

Thrust overcomes gravity which sounds dodgy.

 

[Parvee]

Thrust overcomes gravity which sounds dodgy.

Yeah it isn't possible but in theory it would and all you need to talk about is how escape velocity is the initial velocity required to escape the gravitational field and how the velocity would decrease as the spaceship travels whilst with an engine the spaceship can achieve a lower escape velocity due to it being capable of traveling at a constant velocity.

 

[someth1ng]

The question isn't very clear, it suggests that the engine stays on and in that case, it's technically possible...but still, poor question, in my opinion.

 

[SpiralFlex]

The question isn't very clear, it suggests that the engine stays on and in that case, it's technically possible...but still, poor question, in my opinion.

Hence why it is a Catholic paper.

 

[Parvee]

New Question time!
In his explanation of the Special Theory of Relativity, Einstein used a thought experiment involving a passenger on a train looking into a mirror held out in front of them while the train approached, and then reached, the speed of light. Einstein stated that, without relativity, the image of the person in the mirror would disappear once the train reached light speed.
Using relativity, describe what problems there are with the above thought experiment and how Einstein described what would happen in reality.

 

[barbernator]

New Question time!
In his explanation of the Special Theory of Relativity, Einstein used a thought experiment involving a passenger on a train looking into a mirror held out in front of them while the train approached, and then reached, the speed of light. Einstein stated that, without relativity, the image of the person in the mirror would disappear once the train reached light speed.
Using relativity, describe what problems there are with the above thought experiment and how Einstein described what would happen in reality.

Einstein suggested that in reality, at the speed of light, c, his reflection would still be observable, yet also the light would still be travelling at c relative to him. As Einstein believed that the laws of special relativity could not be violated in any situation, the speed of light must remain constant in all inertial frames of reference relative to any observer, because if the image was not observable due to the light never returning to the observer, or even if light slowed down, he would be performing a mechanical experiment to determine whether he was moving at a constant speed, or stationary.