The particles and the forces on them
Type the label:
Connected particles over a table, on a slope, hanging either side of a pulley, on two slopes, in a line (a car towing a caravan, particles joined by a string or a rod) and in lifts: choose the masses, angles, forces and friction, release the particles, label and resolve the forces, and step through finding the direction of motion, the acceleration, the tension, the reaction and the other unknowns.