# whats wrong with this? [duplicate]

a car weighs 100kg and has Velocity 0. it accelerates and its speed become 10m/s. its kinetic energy would be 100*10*10/2=5000j.

from another view car is at rest and the world accelerates and has now 10m/s speed.

as energy must be constant it should be:5000=10*10/2*(world mass except car) world mass except car=100kg?

am i using conservation of energy in wrong way, or i cant have that point of view?

## marked as duplicate by David Z♦Dec 30 '16 at 10:20

The kinetic energy you determine objects to have depends on what reference frame you're in. Therefore, there's no problem here.

• so it means i cant change my point of view and expect the same 5000j? – Nemexia Dec 30 '16 at 7:43
• That is correct. – probably_someone Dec 30 '16 at 7:43

Energy is dependent on the coordinate frame, so in one frame the car has $E=5000J$ and in another it has $E=0$, nothing wrong about it! This is because energy is not a scalar but a component of a vector, the momentum 4-vector

$$p^\mu=(E, \vec p)$$

It transforms with a Lorentz matrix, like the coordinate 4-vector.

• so it means i cant change my point of view and expect the same 5000j? – Nemexia Dec 30 '16 at 7:44
• Exactly! For some quantities you can, those are called scalars. But for energy you cannot, since energy is not a scalar but a component of a vector. – Photon Dec 30 '16 at 8:19