Moment of force. The lever and its equilibrium condition
A lever is a rigid body (a stick, a plank) that can turn about a fulcrum. The arm l of a force is the shortest distance from the fulcrum to the line of action of the force. The moment of a force is the force times its arm: M = F · l, in N·m. The turning effect is determined by the moment: the longer the arm, the smaller the force needed. Equilibrium condition of a lever: the moment turning clockwise equals the moment turning anticlockwise, F₁ · l₁ = F₂ · l₂, or F₁ : F₂ = l₂ : l₁. So a lever that gives a gain in force by some factor loses the same factor in distance. A saying is attributed to Archimedes: “Give me a fulcrum and I will move the Earth.” Examples of levers: scissors, pliers, a wheelbarrow, scales, a crowbar, a spade, a door handle. A door is easier to push far from the hinges than near them, because the arm is longer.
“Seesaw balance”: place a 1 m ruler on a pencil and find balance with coins on both arms: one coin far from the fulcrum balances two coins closer. Safety: do not jump on a real seesaw; experiment only under adult supervision.