# Energy in different references

1. Jan 4, 2010

### hisham.i

I was thinking in special relativity and how does the energy look from one reference to another,
if i consider a body making a circular motion in reference R1, with an angular velocity w1,and a stationary body in reference R2, R1 see that the body in R2 is rotating with an angular velocity |w1|, so the body in reference R2has a kinetic energy 1/2(I*W1^2) from reference R2point of view.
But the idea which is puzzling me is: Is this kinetic energy really exist?!

2. Jan 4, 2010

### tiny-tim

Hi hisham.i!

Are you using the rotating body in R1 as a reference frame?

You can't do that in special relativity …

all frames must have uniform velocity and be non-rotational.

3. Jan 4, 2010

### Fredrik

Staff Emeritus
Tiny-tim, that's not correct. You can use any coordinate system you want as long as you don't call it an inertial frame.

hisham.i, your question doesn't have anything to do with relativity. You have the same issue in Newtonian mechanics. The answer is yes, the kinetic energy is different in different coordinate systems.

4. Jan 4, 2010

### tiny-tim

Hi Fredrik!
Yes, I agree, but I meant frames "in special relativity", by which I took hisham.i to mean frames related by the Lorentz transformation.

5. Jan 4, 2010

### hisham.i

Yes you are right..
but the point is can i really use this energy in the other reference i mean can i generate electricity from this rotation?

6. Jan 5, 2010

### Ikoro

yes, everything has energy so far it once had a rest mass..or relativistic mass