SUMMARY

This unit's key results, gathered together:

• Aristotle held that force is needed to maintain motion; Galileo (and Newton) showed force is needed only to change motion. Mass measures inertia.
• Newton's first law: with no external force, a body's state of motion is unchanged (this law defines inertial frames).
• Newton's second law: $\vec F=d\vec p/dt$ (valid only in inertial frames); for constant mass, $\vec F=m\vec a$.
• Newton's third law: every force has an equal, opposite reaction, acting on the other body.
• A free body diagram isolates one object's own forces (never the forces it exerts elsewhere).
• The total momentum of an isolated system is conserved; internal forces cannot change it.
• Lami's theorem relates three concurrent, coplanar forces via the sines of their opposite angles.
• Impulse equals the change in momentum, $J=\int F\,dt=\Delta p$.
• Static friction ranges from $0$ to $\mu_sN$; once sliding, kinetic friction is $\mu_kN$ ($\mu_k<\mu_s$); rolling friction is smaller still. …