Is Kinetic engergy relative to an observer?

[mceddy2001]

Since all motion is relative to an inertial frame of reference, is a bodies kenetic energy only relevant when the observer is the intertial frame of reference? (i.e. would the kinetic energy be different for another observer and if so does this violate any of the principles of energy conservation or relativity?

This is not for a homework question this is just out of interest.

Me and my physics teacher think this is an interesting topic. Because motion is relative, is speed simply just an illusion created by an observer? If you a traveling at the speed of light in an empty universe (you would be stationary using urself as an inertial frame of reference) so would it be possible to speed up even faster. Since there would be nothing to say your speed is relative to, then speed really is an illusion.

Please answer any of the questions asked or contiue the converstation by adding ideas. Any arguments or criticisms to my statements above are welcome.

 

[LURCH]

Originally posted by Ambitwistor

It would violate the laws of relativity for an observer to travel at the speed of light. (Massless objects such as photons aren't really "observers"; their worldlines are not timelike.) The laws of relativity also say that any object that travels at the speed of light is massless, and massless objects can only travel at the speed of light.

But it would also violate relativity for an object that is alone in a universe of its own to be traveling at all. If, as you have mentioned, speed is relative to an observer, and you are the only observer, then no matter what, you're stationary (in any inertial frame).

 

[mceddy2001]

since heat is to do with the motion of a particle (vibrations) could heat be relative, would the speed of the Earth suddenly changing cause a fluctuation of tempereatures all over the world, or have i just made a question up on complete lack of back ground knowledge?

 

[Rocksicle]

What happens if you have two objects (or observers, or people) moving away from each other at half the speed of light? Does this mean that one object is moving at the speed of light relative to the other? Would either observer be able to see the other?

 

[jcsd]

Originally posted by Rocksicle
What happens if you have two objects (or observers, or people) moving away from each other at half the speed of light? Does this mean that one object is moving at the speed of light relative to the other? Would either observer be able to see the other?

No they would not see each other moving at the speed of light, they'd actually see each other moving at 0.8c (v = (u + v')/(1 + uv'/c²).

 

[turin]

Originally posted by mceddy2001
Since all motion is relative to an inertial frame of reference, is a bodies kenetic energy only relevant when the observer is the intertial frame of reference?

While kinetic energy is frame dependent (practically by its very definition), a change in kinetic energy is consistent from one inertial frame to another. Do a calculation for a colission between two massive bodies that is not perfectly elastic. You will find that the loss of kinetic energy is the same in any frame that you perform the calculation (I'm pretty sure of this, but not absolutely positive. I have done the calculation myself for a perfectly inelastic collision, because I had the same concern about kinetic energy).

 

[turin]

Originally posted by LURCH
But it would also violate relativity for an object that is alone in a universe of its own to be traveling at all. If, as you have mentioned, speed is relative to an observer, and you are the only observer, then no matter what, you're stationary (in any inertial frame).

I don't think it violates relativity, but I do agree that motion doesn't make sense in this HIGHLY HYPOTHETICAL situation.