Glossary — Forces and motion
Acceleration — the rate of change of velocity, found from the change in velocity divided by the time taken. Acceleration is measured in metres per second squared.
Atmospheric pressure — the pressure caused by air molecules colliding with a surface. It decreases as the height above the ground increases.
Average speed — the total distance travelled divided by the total time taken, for a journey in which the speed changes.
Braking distance — the distance a vehicle travels while the brakes are acting.
Centre of mass — the single point at which the weight of an object may be considered to act.
Compression — the decrease in the length of an object when it is pushed.
Conservation of momentum — the principle that in a closed system the total momentum before an event equals the total momentum after it.
Contact force — a force that acts when two objects touch, such as friction, air resistance, tension or the normal contact force.
Deceleration — the rate at which an object slows down; a negative acceleration.
Density — the mass of one cubic metre of a substance, measured in kilograms per cubic metre.
Distance — how far an object moves. Distance has no direction, so it is a scalar.
Drag — the resistive force on an object that moves through a fluid.
Elastic deformation — a change of shape in which an object returns to its original shape and size when the forces are removed.
Extension — the increase in the length of an object when it is stretched, measured in metres.
Fluid — a liquid or a gas.
Force — a push or a pull that acts on an object because of an interaction with another object. Force is a vector, measured in newtons.
Free body diagram — a diagram that shows one object on its own, with an arrow for every force that acts on it.
Gear — a wheel with teeth. Two gears that mesh transmit the turning effect of a force from one to the other.
Gravitational field strength — the force of gravity on each kilogram of an object, measured in newtons per kilogram. On the Earth it is about 9.8 N/kg.
Inelastic deformation — a change of shape in which an object does not return to its original shape and size when the forces are removed.
Inertia — the tendency of an object to stay at rest, or to keep moving at a constant velocity.
Inertial mass — a measure of how difficult it is to change the velocity of an object, defined as the force divided by the acceleration it produces.
Lever — a bar that turns about a pivot and transmits the turning effect of a force. A lever can multiply a force.
Limit of proportionality — the point on a force-extension graph beyond which the extension is no longer proportional to the force.
Mass — the amount of matter in an object, measured in kilograms. Mass does not change when the object moves to another place.
Moment — the turning effect of a force, found from the force multiplied by the perpendicular distance from the pivot. The unit is the newton-metre.
Momentum — the product of the mass of an object and its velocity, measured in kilogram metres per second. Momentum is a vector.
Newton — the unit of force, written N.
Newton's first law — if the resultant force on an object is zero, a stationary object stays stationary and a moving object keeps a constant velocity.
Newton's second law — the acceleration of an object is proportional to the resultant force and inversely proportional to the mass of the object, so force = mass × acceleration.
Newton's third law — whenever two objects interact, the forces they exert on each other are equal and opposite.
Newtonmeter — a spring balance with a scale, used to measure a force in newtons.
Non-contact force — a force that acts between objects that are apart, such as the gravitational force, the electrostatic force or the magnetic force.
Normal contact force — the push from a surface on an object, at right angles to the surface.
Pivot — the fixed point about which an object turns.
Pressure — the force normal to a surface divided by the area of the surface, measured in pascals. One pascal is one newton per square metre.
Reaction time — the time between noticing a hazard and starting to act. For a driver, the time between seeing a hazard and braking.
Resultant force — a single force that has the same effect as all the forces acting on an object together.
Scalar — a quantity that has a size but no direction, such as mass or speed.
Speed — how fast an object moves. Speed has no direction, so it is a scalar.
Spring constant — the force needed to stretch a spring by one metre, measured in newtons per metre. A larger spring constant means a stiffer spring.
Stopping distance — the total distance a vehicle travels from the moment the driver sees a hazard to the moment it stops; the thinking distance plus the braking distance.
Terminal velocity — the steady velocity of an object falling through a fluid. It is reached when the drag force equals the weight, so the resultant force is zero.
Thinking distance — the distance a vehicle travels while the driver reacts.
Upthrust — the resultant upward force on an object in a fluid, caused by the greater pressure on its lower surface.
Vector — a quantity that has both a size and a direction, such as force.
Velocity — speed in a given direction. Velocity is a vector.
Weight — the force of gravity on an object, measured in newtons. Weight is mass multiplied by gravitational field strength.