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# 15.22: Momentum

[ "article:topic", "authorname:tatumj" ]

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.