Newton's laws, free-body diagrams, and every kind of force you'll meet all year — friction, springs, gravity, drag, and circular motion — all traced back to ΣF = dp/dt.
Episode Guide — Season 2
A system can be treated as a single point at its center of mass. For continuous objects, that point comes from an integral, not just an average.
Every force is an interaction between two objects. Draw them as vectors from the center-of-mass dot, and the algebra practically writes itself.
Every force comes with a partner, equal and opposite, acting on the other object. Tension and ideal pulleys make this precise.
Zero net force means constant velocity — nothing more, nothing less. Balanced in one direction says nothing about what's happening in another.
F = ma is the special case. The real law is that net force is the rate of change of momentum — and that version survives everything Unit 4 throws at it.
Every mass pulls on every other mass. Near Earth's surface that collapses into mg — but the inverse-square law underneath never goes away.
Static friction resists up to a maximum; kinetic friction is (almost) constant once things start sliding. Knowing which one applies changes the whole problem.
Hooke's law is a restoring force — always pointed back toward equilibrium, always proportional to displacement. This is where oscillations start.
Drag depends on velocity itself, which means the equation of motion becomes a differential equation. Terminal velocity falls out when acceleration finally hits zero.
Constant speed doesn't mean constant velocity. Center-seeking acceleration keeps an object turning even when nothing speeds it up or slows it down.