Inertial reference frame.

Equilibrium.

Momentum form.

$$ \mathbf{F}_{12} = \frac{Gm_1m_2}{r^2} \hat{r}_{12} $$

$$ \mathbf{g} = \frac{GM}{r^2} \hat{r} $$

Einstein

Inertial mass

$$ m_i = \frac{F}{a}. $$

Gravitational mass

$$ m_g = \frac{F_g}{g}. $$

Using free-body diagrams without friction.