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Physics LibreTexts

15.22: Momentum

The linear momentum \( \bf{p}\) of a body, referred to a frame \( \Sigma\), is defined as

\[ \bf{p} = m\bf{u}. \label{15.22.1}\]

Here \( m\) and \( \bf{u}\) are its mass and velocity referred to \( \Sigma\). Note that \( m\) is not the rest mass.

Example \(\PageIndex{1}\)

The rest mass of a proton is 1.67 % 10-27 kg. What is its momentum referred to a frame in which it is moving at 99% of the speed of light?

Answer = 3.51 % 10-18 kg m s-1.

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