NCEA Level 3 Physics key terms

The rate at which velocity changes with time, including direction. It is a vector quantity measured in metres per second squared, and is non-zero whenever speed or direction changes.

The rate of change of displacement with time, including direction. Unlike speed it is a vector, so a body moving in a circle at constant speed still has changing velocity.

The straight-line distance and direction from a starting point to a finishing point. It is a vector, so it can be zero even when the total distance travelled is large.

A quantity that has both magnitude and direction, such as force, velocity or momentum. Vectors are added tip-to-tail or resolved into perpendicular components.

A quantity that has magnitude only, with no direction, such as mass, time, speed, energy or temperature. Scalars add arithmetically and cannot be resolved into components.

The part of a vector acting along a chosen axis, found by multiplying the magnitude by the sine or cosine of the angle to that axis. Perpendicular components are treated independently.

Splitting a single vector into two perpendicular components, usually horizontal and vertical. This lets motion in each direction be analysed separately, for example projectile motion.

The net force on an object equals its mass times its acceleration, written F = ma. The acceleration is always in the direction of the net force acting on the object.

The single vector sum of all forces acting on an object. It determines the object's acceleration; when it is zero the object stays at rest or moves at constant velocity.

A diagram showing one object in isolation with every force acting on it drawn as an arrow from its centre, labelled with magnitude and direction. Used to find the net force.

The gravitational force on an object, equal to its mass multiplied by the gravitational field strength, W = mg. It acts vertically downwards toward the centre of the attracting body.

A measure of how much matter an object contains and of its resistance to acceleration. It is a scalar measured in kilograms and does not change with location.

The tendency of an object to resist any change in its state of motion. Objects with greater mass have greater inertia, needing a larger net force for the same acceleration.

An object remains at rest or moves at constant velocity unless acted on by a net external force. It defines inertia and explains why passengers lurch forward when a bus stops.

For every action force there is an equal and opposite reaction force, acting on a different body. The pair never cancels because the two forces act on separate objects.

The state of a body when the vector sum of all forces on it is zero, so it has zero acceleration. It may be at rest or moving with constant velocity.

A contact force that opposes relative motion or the tendency to move between surfaces. It acts along the surface and converts kinetic energy into heat and sound.

The perpendicular contact force a surface exerts on an object resting against it. It adjusts to balance other forces and is not always equal to the object's weight.

Motion of an object moving under gravity alone, with constant horizontal velocity and constant vertical acceleration. The two components combine to give a parabolic path.

Motion along a circular path at constant speed, with velocity continually changing direction. It requires a net force directed toward the centre called the centripetal force.

The net force directed toward the centre of a circular path that keeps an object moving in that circle. It equals mv squared divided by the radius.

The turning effect of a force, equal to the force multiplied by the perpendicular distance from the pivot. It is measured in newton metres and causes angular acceleration.

The rate of change of angular velocity, measured in radians per second squared. It is produced by a net torque and equals torque divided by the rotational inertia.

An object's resistance to change in its rotation, depending on its mass and how that mass is distributed about the axis. Often called the moment of inertia.

The product of an object's mass and velocity, a vector measured in kilogram metres per second. It changes only when a net force acts over time.

The product of a force and the time for which it acts, equal to the change in momentum. Extending the contact time reduces the force for the same momentum change.

In a closed system with no net external force, total momentum before an interaction equals total momentum after. It applies to collisions and explosions in any direction.

A collision in which both momentum and kinetic energy are conserved. Objects bounce apart with no loss of kinetic energy to heat, sound or deformation.

A collision in which momentum is conserved but kinetic energy is not, because some is converted to heat, sound or deformation. Objects may stick together and move as one.

The transfer of energy when a force moves an object through a distance, calculated as force times displacement in the force's direction. Measured in joules.

The energy an object has because of its motion, equal to half its mass times its speed squared. It is a scalar measured in joules and is always positive.

Energy stored in an object because of its position in a gravitational field, near Earth equal to mass times gravity times height. It can be converted to kinetic energy as the object falls.

Energy cannot be created or destroyed, only transformed between forms or transferred between objects. The total energy of a closed system stays constant.

The rate at which energy is transferred or work is done, equal to energy divided by time. It is measured in watts, where one watt is one joule per second.

The ratio of useful energy or power output to total energy or power input, often expressed as a percentage. It is always less than one because some energy is dissipated.

The gravitational force per unit mass at a point, measured in newtons per kilogram. Near Earth's surface it is about 9.8 and points toward Earth's centre.

Oscillation in which acceleration is proportional to displacement from equilibrium and directed toward it. It produces sinusoidal displacement, velocity and acceleration graphs.

The time taken for one complete cycle of a repeating motion or wave, measured in seconds. It is the reciprocal of frequency.

The number of complete cycles of a periodic motion or wave that occur each second, measured in hertz. It equals one divided by the period.

The large-amplitude response that occurs when a system is driven at its natural frequency. Energy is absorbed most efficiently and amplitude peaks sharply.