I'm hoping that someone can help me understand a topic on page 86 of Feynman's "Six Not-so-easy-pieces", in the "4-4 Relativistic mass" section. He says, "It is an interesting exercise to now check whether or not Eq.(4.9) is indeed true for arbitrary values of w, assuming that Eq.(4.10) is the right forumula for the mass." http://www.feynmanlectures.caltech.edu/I_16.html#Ch16-S4(adsbygoogle = window.adsbygoogle || []).push({});

I can follow the derivations of (4.9,10), but I don't understand how to perform the exercise. My best result was to use

$$ v^2 = u^2 + w^2(1 - u^2/c^2)$$

to expand (4.9) to

$$\frac{m_w}{m_v} = \sqrt{1 - \frac{w^2 + v^2}{c^2 - w^2}}$$

which becomes (4.10) in the limit of the vertical components approaching zero. But I don't think that's the exercise Feynman was proposing. Any ideas?

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# Relativistic mass in Six Not-so-easy Pieces (Feynman)

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