什么是 Motor, resistive, zero work？

The sign of $\cos\theta$ sorts every force into three regimes:

• $\theta < 90{}^{\circ}$: $W > 0$, the work is motor — the force feeds energy into the motion;
• $\theta = 90{}^{\circ}$: $W = 0$, the force works not at all, however large;
• $\theta > 90{}^{\circ}$: $W < 0$, the work is resistive — the force drains energy. The pure case is a brake at $180{}^{\circ}$: $800\,\mathrm{N}$ over $45\,\mathrm{m}$ takes $800 \times 45 \times (-1) = -36\,\mathrm{kJ}$ out of a car, into hot discs.