3. Newton's laws

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.