Every key term, plainly defined.

A

Absolute Zero

The lowest possible temperature, 0 K (−273.15 °C), at which particle thermal motion is at a minimum.

Acceleration Learn more

The rate at which velocity changes over time, measured in m/s². A vector quantity.

Amplitude

The maximum displacement of a wave or oscillation from its equilibrium (rest) position.

Angular Velocity Calculator

The rate at which an object rotates about an axis, measured in radians per second.

Atom

The smallest unit of an element that keeps its chemical identity, made of protons, neutrons, and electrons.

B

Boyle's Law Calculator

For a fixed amount of gas at constant temperature, pressure is inversely proportional to volume.

Buoyancy Calculator

The upward force a fluid exerts on an immersed object, equal to the weight of fluid it displaces.

C

Capacitance

A measure of how much electric charge a component stores per unit voltage, in farads.

Centripetal Force Calculator

The net inward force that keeps an object moving along a circular path.

Conduction

Heat transfer through direct contact between particles, with no bulk movement of matter.

Conservation of Energy Learn more

Energy cannot be created or destroyed, only transformed from one form to another.

Conservation of Momentum Calculator

The total momentum of an isolated system stays constant when no external force acts.

Convection

Heat transfer by the bulk movement of a heated fluid (a liquid or gas).

Coulomb's Law Learn more

The electrostatic force between two charges is proportional to their product and inversely proportional to the square of the distance between them.

Current

The rate of flow of electric charge through a conductor, measured in amperes (A).

D

Density

The mass per unit volume of a substance, measured in kg/m³.

Diffraction

The bending and spreading of waves as they pass an edge or move through an opening.

Displacement Learn more

The straight-line distance and direction from start point to end point; a vector quantity.

Doppler Effect Calculator

The change in observed frequency of a wave when the source and observer move relative to each other.

E

Elastic Collision

A collision in which both total momentum and total kinetic energy are conserved.

Electric Field

A region around a charge in which another charge feels a force; measured in N/C or V/m.

Energy Learn more

The capacity to do work, measured in joules (J). It comes in many forms but is always conserved.

Entropy

A measure of disorder, or of energy unavailable for work, in a system; it always increases in an isolated system.

Equilibrium

A state in which all forces and torques on an object are balanced, so there is no net acceleration.

Escape Velocity Calculator

The minimum speed an object needs to escape a body's gravity without further propulsion.

F

Force Learn more

A push or pull that can change an object's motion, measured in newtons (N).

Free Fall Calculator

Motion under the influence of gravity alone, with no other forces such as air resistance.

Frequency Learn more

The number of wave cycles or oscillations passing a point per second, measured in hertz (Hz).

Friction Learn more

A force that opposes the relative motion or attempted motion of surfaces in contact.

G

Gravitational Potential Energy Calculator

Stored energy due to an object's height in a gravitational field, U = mgh.

Gravity

The attractive force between any two masses. Near Earth's surface it gives an acceleration g ≈ 9.8 m/s².

H

Half-Life

The time it takes for half the nuclei in a radioactive sample to decay.

Heat Learn more

Energy transferred between objects because of a temperature difference between them.

Hooke's Law Calculator

The extension of a spring is proportional to the force applied, F = kx (within its elastic limit).

I

Ideal Gas Law Calculator

Relates the pressure, volume, and temperature of a gas: PV = nRT.

Impulse Calculator

The product of a force and the time over which it acts; it equals the change in momentum.

Inertia Learn more

An object's resistance to changes in its state of motion, related to its mass.

Interference

The combining of two or more waves to produce a resultant of greater, lower, or equal amplitude.

J

Joule

The SI unit of energy and work, equal to one newton-metre (N·m).

K

Kinematics Calculator

The study of motion — position, velocity, and acceleration — without reference to the forces that cause it.

Kinetic Energy Learn more

The energy an object has because of its motion, KE = ½mv².

L

Latent Heat

The energy absorbed or released during a change of state (phase change) at constant temperature.

Longitudinal Wave

A wave in which particle motion is parallel to the direction of energy travel, such as sound.

M

Magnetic Field

A region in which a magnetic force acts on moving charges and magnetic materials.

Mass

The amount of matter in an object and a measure of its inertia, in kilograms (kg).

Momentum Calculator

The product of an object's mass and velocity (p = mv); a conserved vector quantity.

N

Newton

The SI unit of force — the force that gives a 1 kg mass an acceleration of 1 m/s².

Newton's First Law Learn more

An object stays at rest, or moves at constant velocity, unless acted on by a net external force.

Newton's Second Law Learn more

The net force on an object equals its mass times its acceleration, F = ma.

Newton's Third Law Learn more

For every action there is an equal and opposite reaction.

Normal Force

The support force a surface exerts perpendicular to an object resting on it.

O

Ohm's Law Calculator

For many conductors, voltage equals current times resistance, V = IR.

Optics

The branch of physics dealing with the behaviour and properties of light.

P

Period Calculator

The time taken for one complete cycle of an oscillation or wave, measured in seconds.

Photon

A quantum (particle) of electromagnetic radiation, carrying energy E = hf.

Potential Energy Learn more

Stored energy due to an object's position or configuration.

Power Calculator

The rate at which work is done or energy is transferred, measured in watts (W).

Pressure

Force applied per unit area, measured in pascals (Pa).

Projectile Motion Learn more

The curved, parabolic path of an object launched into the air and moving under gravity.

Q

Quantum

The smallest discrete amount of a physical quantity, such as a single packet of energy.

R

Radiation

The transfer of energy as electromagnetic waves or particles, requiring no medium.

Reflection

The bouncing back of a wave when it meets a boundary between two media.

Refraction Calculator

The bending of a wave as it passes from one medium into another of different density.

Resistance Calculator

A measure of how strongly a material opposes electric current, measured in ohms (Ω).

Resonance

A large-amplitude response that occurs when a system is driven at its natural frequency.

S

Scalar

A quantity that has magnitude but no direction, such as mass, time, or temperature.

Snell's Law Calculator

Relates the angles of incidence and refraction to the refractive indices of two media.

Special Relativity Learn more

Einstein's theory describing space, time, and motion for objects moving at high speeds.

Specific Heat Capacity Learn more

The energy needed to raise the temperature of 1 kg of a substance by 1 °C, Q = mcΔT.

Speed Learn more

How fast an object moves regardless of direction; a scalar quantity.

Static Electricity

An imbalance of electric charge built up on the surface of a material.

T

Temperature Learn more

A measure of the average kinetic energy of the particles in a substance.

Tension

The pulling force transmitted along a string, rope, cable, or similar object.

Thermal Expansion Calculator

The tendency of matter to change in size in response to a change in temperature.

Torque Calculator

The rotational equivalent of force — the turning effect produced about an axis.

Transverse Wave

A wave in which particle motion is perpendicular to the direction of travel, such as light.

U

Uniform Motion

Movement at a constant velocity — constant speed in a straight line.

V

Vector

A quantity that has both magnitude and direction, such as velocity, force, or displacement.

Velocity Learn more

The rate of change of displacement; speed together with a direction. A vector quantity.

Voltage Calculator

The electric potential difference that drives current through a circuit, measured in volts (V).

W

Wavelength Calculator

The distance between successive identical points on a wave, such as crest to crest.

Wave Speed Calculator

How fast a wave travels through a medium, given by v = fλ.

Weight

The gravitational force acting on an object, W = mg, measured in newtons (N).

Work Calculator

Energy transferred when a force moves an object through a distance, W = Fd.

X

X-ray

A form of high-energy electromagnetic radiation with a very short wavelength.

Y

Young's Modulus

A measure of a material's stiffness — the ratio of stress to strain in the elastic region.

Z

Zeroth Law of Thermodynamics

If two systems are each in thermal equilibrium with a third, then they are in equilibrium with each other.