Glossary — Mechanics
Acceleration — the rate of change of velocity.
Apparent weight — the normal reaction from the floor on a person in a lift; it is what the person feels.
Centre of mass — the point at which the weight of a body acts.
Coefficient of friction — the constant in the friction model .
Constant-acceleration formulae — the four equations that link initial velocity, final velocity, acceleration, displacement and time when acceleration is constant.
Derived quantity — a quantity built from the fundamental quantities, with a unit that comes from its formula.
Displacement — the change in position; a vector.
Distance travelled — the total length of the path, whatever the direction; a scalar.
Equilibrium — the state of a body when the resultant force on it is zero.
Force — a push or a pull; a vector measured in newtons.
Force diagram — a diagram that shows one chosen body on its own, with an arrow for every force acting on it.
Free fall — motion under gravity alone, with no air resistance.
Friction — a contact force between two surfaces that opposes sliding, or the tendency to slide.
Fundamental quantity — a quantity that is defined directly and is not built from another, such as length, mass or time.
Mass — the amount of matter in a body, measured in kilograms; it does not change with place.
Moment — the turning effect of a force about a point, found from the force multiplied by the perpendicular distance.
Newton's first law — a body stays at rest, or keeps moving with a constant velocity, unless a resultant force acts on it.
Newton's second law — the resultant force on a body equals its mass multiplied by its acceleration, .
Newton's third law — if body A exerts a force on body B, then B exerts a force of the same size on A, in the opposite direction.
Normal reaction — the push from a surface at right angles to that surface.
Principle of moments — for a body in equilibrium, the sum of the clockwise moments about a point equals the sum of the anticlockwise moments.
Projectile — an object moving under gravity alone, with no air resistance.
Resultant force — a single force that has the same effect as all the forces acting on a body together.
SI system — the international system of units used in science, built from the base units of length, mass and time.
Sign convention — the choice of which direction is positive, used to give signs to vector quantities in kinematics.
Speed — the rate at which distance is covered; a scalar.
Tension — the pull in a string or rope.
Terminal velocity — the constant velocity reached by a falling body when the resultant force on it is zero.
Uniform rod — a rod whose mass is spread evenly, so its centre of mass is at its midpoint.
Velocity — the rate of change of displacement; a vector.
Weight — the force of gravity on a body, found from .