We can measure kinetic energy by two formulas..- K= and by K= F.S
If we have two frames S and S' and a particle is moving with speed V in a particular direction.If S' moves with the same velocity in the same direction,we are able to measure zero as well as non zero K.E from inertial frames.
However, if we use the definition K.E.=F.S,a problem arises.Let us suppose that two scientists at frames S and S' are given tha value of the applied force.now,the scientist at S' would want to measure zero kinetic energy fot the same particle.To do so,after the force has been initiated he would have to cover the same displacement within which the particle stops at the same time.to do so would mean that both the particle as well as the frame will get accelerated meaning that S' would become NON-INERTIAL.
Thus,if we use the former formula of K.E we can get zero K.E value for a particle within inertial frames.However to do so with the later formula,we would not be able to get zero energy from an inertial frame
