Suppose a ball is travelling at 5km/h, person A is travelling at 2km/h and person B is travelling at 5km/h in the opposite direction. The ball is travelling at 3km/h relative to A and at 10km/h relative to B. If both calculate the kinetic energy of the ball then both would get different value for the kinetic energy(1/2 mv2). Does this mean that energy can be relativistic?
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asked Jan 17, 2019 at 10:07
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5$\begingroup$ Possible duplicate of If movement is relative, why is it said that moving objects have kinetic energy? $\endgroup$– Kyle OmanCommented Jan 17, 2019 at 10:12
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$\begingroup$ Duplicate/related?Is the change in kinetic energy of a particle frame independent? $\endgroup$– FarcherCommented Jan 17, 2019 at 10:27
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2 Answers
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Yes, it can be relative. Energy sum needs to be preserved in any means of refrence frames. If that ball kicks you in the head, it will hurt the same if you calculate you movimg towards ball 5kmh and ball going towards you at 5kmh, or simply ball kicking you with 10kmh.
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The velocity of an object, and therefore its kinetic energy, depends on the reference frame in which it is measured. If you are in a car of mass $m$ traveling at velocity $v$ with respect to the road, the car's kinetic energy in your frame of reference is zero, whereas it is $\frac{mv^2}{2}$ in the frame of reference of a person standing on the road. Big difference for the person on the road!
So yes, the kinetic energy of an object is relative.
Hope this helps.
