Kinetic Energy and relative momentum

[jlennoxg]

I've just started getting interested in physics and I am only a few (basic) books deep but there is something that has been distracting me that I can't come to a logical conclusion about. I assume this means there is a flaw in my basic understanding and wish to fix it asap. Here goes.

I am assuming the following statements to be true (as is my understanding which, as I've said, is likely flawed).

*The kinetic energy of an object is greater the faster it is moving.

*As motion is relative, Person A on the ground can observe Person B whizzing through space close to the speed of light and claim that they are stationary and it is Person B who is moving. Likewise, Person B can claim that they are stationary and it is indeed Person A who is moving close to the speed of light (along with the Earth which Person A is standing on).

What I don't understand is where kinetic comes into this. If Person A is stationary they have no kinetic energy and they perceive Person B to have a high amount of kinetic energy. But at the same time Person B perceives them to have a high kinetic energy similar to that which Person A thinks Person B has. They can't both have high kinetic energy AND no kinetic energy can they?

I hope that makes sense and I've not overcomplicated it too much. Sorry if this is a stupid question or has an obvious answer, it's just something that has been bugging me! Thanks.

 

[A.T.]

If Person A is stationary they have no kinetic energy and they perceive Person B to have a high amount of kinetic energy. But at the same time Person B perceives them to have a high kinetic energy similar to that which Person A thinks Person B has. They can't both have high kinetic energy AND no kinetic energy can they?

Kinetic energy is frame dependent. It is not an intrinsic property of the object. It is a number assigned to the object based on the reference frame, just like position and velocity.

Consequently the transfer of KE is also frame frame dependent:
http://www.youtube.com/watch?v=fXr3O1cxYrc#t=90s

If you internalize those facts, you will never be confused asking yourself where the energy comes from to drive one of these: