A body is moving with a velocity $v$ with respect to a frame of reference $S_1$.
It bumps into a very heavy object and comes to rest instantaneously, its kinetic energy
$$\frac{1}{2}mv^2$$
as seen from the frame $S_1$ is completely converted to thermal energy.
Now, a man moving with a uniform velocity $V$ (in the direction of the body) with respect to $S_1$ observes the body. He notes that its initial kinetic energy of the body is
$$\frac{1}{2}m(v+V)^2,$$
and that after it rams into the heavy body as
$$\frac{1}{2}mV^2,$$
and concludes that the thermal energy produced is the difference
$$\frac{1}{2}m(v+V)^2-\frac{1}{2}mV^2~=~\frac{1}{2} mv^2+mvV.$$
Which of the two answers is correct and why?
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