====== 3. Newton's laws ======
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===== - Force =====
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===== - First law =====

Inertial reference frame.

Equilibrium.
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===== - Second law =====
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Momentum form.

===== - Third law =====
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===== - Law of gravitation =====
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$$ \mathbf{F}_{12} = \frac{Gm_1m_2}{r^2} \hat{r}_{12} $$

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

Einstein
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===== - Inertial and gravitational mass =====
Inertial mass

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

Gravitational mass

$$ m_g = \frac{F_g}{g}. $$
===== - Application of Newton's laws =====
Using free-body diagrams without friction.
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==== Friction ====
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