Laws of Motion — Newton's Laws & Friction (Medium)

Quick Recap — Newton's Laws & Friction

• Newton's second law: $\vec F_{net}=m\vec a$; impulse $\vec J=\vec F\,\Delta t=\Delta\vec p$. Third law: forces come in equal, opposite pairs on different bodies.
• On an incline (angle $\theta$): component along $=mg\sin\theta$, normal $=mg\cos\theta$; frictionless acceleration $=g\sin\theta$.
• Friction: $f_s\le\mu_s N$, kinetic $f_k=\mu_k N$ (with $\mu_k<\mu_s$); the angle of repose satisfies $\tan\theta=\mu_s$.
• Lifts (apparent weight): accelerating up $m(g+a)$, down $m(g-a)$, free fall $0$ (weightless).

Worked mini-example. A 5 kg block on a floor ($\mu=0.3$, $g=10$) needs a force $f=\mu mg=0.3\times5\times10=15$ N to just start sliding.