$$F = ma \qquad W = mg \quad (g \approx 10\ \text{N/kg or } 9.8\ \text{N/kg})$$

• Resultant of two forces at right angles: $R = \sqrt{F_1^2 + F_2^2}$, direction $\tan\theta = \dfrac{F_2}{F_1}$.
• Weight on another body: $W = mg_{\text{body}}$; mass never changes.

Momentum and impulse

$$p = mv \qquad \text{Impulse} = Ft = \Delta p = mv - mu \qquad F = \frac{mv - mu}{t}$$

Conservation of linear momentum

$$m_1u_1 + m_2u_2 = m_1v_1 + m_2v_2 \quad\text{(total momentum before = after)}$$

• Elastic collision: kinetic energy is also conserved.
• Bodies that join (inelastic): $m_1u_1 + m_2u_2 = (m_1 + m_2)v$.

Friction

$$F_{\text{friction}} = \mu R \qquad \mu = \frac{F}{R}\ (R = \text{normal reaction})$$