You answered that Newton’s Second Law of Motion is not universally valid.

If you thought of the law in the form F = ma, then you are right.

In this form, the law can describe classical mechanics, but it is not generally valid.

Defined as a time derivative of momentum, however, the “Lex Secunda” acquires the character of a universally valid law and thus provides the appropriate expression that accounts for mass change at relativistic velocities:

\[\vec{F} = \frac{d(m\vec{v})}{dt} = m\frac{d\vec{v}}{dt} + \vec{v}\frac{dm}{dt}\]

Please read on. See what Richard Feynman writes about it.

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