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Kinetic Energy and relative momentum

  1. Aug 28, 2011 #1
    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.
  2. jcsd
  3. Aug 28, 2011 #2


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    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:

    If you internalize those facts, you will never be confused asking yourself where the energy comes from to drive one of these:
    Last edited by a moderator: Sep 25, 2014
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