## Roadmap for Formal Mathematical Physics Content

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Suppose an object starts an infinite distance from a moon and is dropped, falling towards the moon due to gravitational acceleration. What is the speed of the object when it is

measure: r distance
symbol: distance $$r$$
from the moon? Figure 1: small mass falling towards a moon from initial position at infinity.

The initial condition is \begin{equation} v(x=\infty) =0 \label{eq:initial_velocity} \end{equation} The force acting on the object is \begin{equation} \vec{F} = \frac{-G m_1 m_2}{x^2} \hat{x} \label{eq:gravitational force} \end{equation} The

measure: W work
is calculated using
symbol: work $$W$$
= $$\Delta E$$ since the force changes. To find the cumulative work done on the object, integrate over all positions between $$\infty$$ and $$r$$ \begin{equation} W = \int_{\infty}^r \vec{F}\cdot d\vec{r} \label{eq:work as function of force} \end{equation} Substituting the gravitational force into Eq. \ref{eq:work as function of force}, \begin{equation} W = \int_{\infty}^r \frac{-G m_1 m_2}{x^2} dx \end{equation} Factor out the constants, \begin{equation} W = -G m_1 m_2\int_{\infty}^r \frac{1}{x^2} dx \end{equation} which leads to \begin{equation} W = \frac{G m_1 m_2}{r} \end{equation} Another definition of
measure: W work
is that it is the
measure: $$\Delta E$$ change in energy
for a system:
symbol: work $$W$$
=
symbol $$\Delta E$$
Because the initial velocity was zero, the work here is \begin{equation} W = \Delta KE \end{equation} Thus we can combine the two definitions of work to get \begin{equation} W = \frac{1}{2} m_1 v^2 = \frac{G m_1 m_2}{r} \end{equation} The $$m_1$$ cancels, leaving \begin{equation} v(r) = \sqrt{\frac{2Gm_2}{r}} \end{equation}