# Conceptual Question about reference frames

1. Jan 10, 2012

### kikko

1. The problem statement, all variables and given/known data

Do objects same kinetic energy in all inertial reference frames?
For objects interacting, is energy conserved in all inertial reference frames?

2. Relevant equations

None

3. The attempt at a solution

I think the answers are No for the first one, and Yes for the second one. Am I right?

2. Jan 10, 2012

### Staff: Mentor

Some people think in terms of billiard balls and consider what happens when they collide.

3. Jan 10, 2012

### kikko

My logic is that since depending on what reference frame you are in, you view different objects at rest, so they'd have different amounts of kinetic energy.

My logic for the 2nd is that intertial reference frames obey the same laws of physics, so energy will be conserved. Am I correct with both responses?

4. Jan 11, 2012

### Staff: Mentor

I think your answers are correct. Here's my logic for the first one:

I'm standing by the road and a car whizzes by at 60mph so its energy is 1/2 m v^2
Next I'm in a car going 60 mph along side the other car so to me his velocity is 0mph hence 0 energy

so I conclude the kinetic energy of an object is relative to my frame of reference

I'll let you consider a thought experiment for the second one.

5. Jan 11, 2012

### Curious3141

Observations of speed, and therefore, kinetic energy, can differ depending on which inertial frame of reference one is considering. This applies in both Galilean (Classical) and Special Relativity.

Energy, momentum, etc. are conserved in all inertial frames of reference. This is also true in both Classical and Special Relativity.