# Page:The Solar System - Six Lectures - Lowell.djvu/25

$\textstyle v^{2}=\mu\left(\frac{2}{r}\mp\frac{1}{a}\right) \,\!$,
Suppose a body at p moving along the curve whose tangent is pt with acceleration f always directed to s. Then $\textstyle \dot v \,\!$, the resolved part of the acceleration along the tangent, is f cos ψ.