48 of the Physics glossary’s terms, each with the part of the syllabus where you first meet it.
A system that sends out microwaves or radio waves and detects their reflections; the Doppler shift of the reflected waves, , gives the speed of the target.
A field whose lines point straight towards (or away from) a single central point, such as the field outside a spherical mass; its strength falls with distance.
The component of a body's velocity along the line of sight, towards or away from the observer; it is the part of the motion that a Doppler shift measures.
The angle at the centre of a circle subtended by an arc equal in length to the radius; a full circle is rad.
The pressure exerted by electromagnetic radiation on matter that absorbs or reflects it, because photons carry momentum.
The random and spontaneous change of an unstable nucleus into a more stable one, with the emission of radiation (α, β or γ).
Unwanted radioactive material, such as spent fuel, that must be isolated from people and the environment until its activity has fallen to safe levels.
Finding the time since an organism died from the activity of carbon-14 per gram of carbon in its remains, which falls with a half-life of 5730 years once intake of carbon stops.
An error that makes readings scatter unpredictably above and below the true value; its effect is reduced by repeating measurements and averaging.
The horizontal distance travelled by a projectile between its launch point and the point where it lands.
A region of a longitudinal wave where the particles of the medium are further apart than normal, so the pressure is lower.
Measurements exactly as they were read from the instruments, before any calculation.
A line showing the direction in which a wave travels and transfers energy; it is always perpendicular to the wavefronts.
The time between a person seeing (or hearing) a signal and starting to respond to it, such as a driver seeing a hazard and pressing the brake.
An actual gas, whose molecules have volume and attract each other, so it deviates from ideal behaviour at high pressure, low temperature and high density.
A large, luminous star with a cool surface, formed when a low-mass star has used up the hydrogen in its core; its main energy source is helium fusion in the core.
An increase in the observed wavelength (and decrease in frequency) of electromagnetic waves from a source moving away from the observer.
A circle of radius round which a point moves at angular speed ; the projection of the point onto a diameter moves in simple harmonic motion, and the angle of its radius is the phase .
A coordinate system, with an origin, axes and synchronised clocks, used to give the position and time of events.
The change in direction of a wave at a boundary so that it returns into the medium it came from, with its speed, frequency and wavelength unchanged.
The change in direction of a wave as it passes from one medium into another in which its speed is different.
The ratio of the speed of light in a vacuum to the speed of light in a medium, . It has no unit and is at least 1.
A heat pump used to keep a space cold: work is done to remove thermal energy from its interior and release it, plus the work, to the surroundings.
The ratio of a medium's permittivity to that of a vacuum; the factor by which the medium reduces the force between charges.
The velocity of one body as measured by an observer moving with another body; for light, only the relative velocity of source and observer along the line joining them sets the Doppler shift.
The rule for the velocity of a body in a frame moving at
Two events at different places that are simultaneous in one inertial frame are not simultaneous in a frame moving along the line joining them.
The chemical separation of spent fuel into reusable uranium and plutonium and the remaining high-level waste.
The focused question an investigation sets out to answer, naming the independent and dependent variables (or two correlated variables) and the system studied.
The ratio of the potential difference across a component to the current through it, ; measured in ohms (Ω).
A property of a material defined by for a sample of resistance , length and cross-sectional area ; measured in Ω m.
The smallest change in a quantity that a measuring instrument can show, such as 1 mm for a metre rule.
The large increase in the amplitude of a forced oscillation when the driving frequency equals, or is very close to, a natural frequency of the system.
A graph of the amplitude of a forced oscillation against the driving frequency, with a peak near the natural frequency.
A force that acts on a displaced body in the direction of its equilibrium position.
The vector sum of all the forces acting on a body: the single force that would have the same effect as all of them together.
The sum of all the torques on a body about one axis, with clockwise and anticlockwise torques given opposite signs.
An idealised process that can be run backwards, returning the system and its surroundings exactly to their original states; the total entropy is unchanged.
A unit of rotation rate used for machines; dividing by 60 gives revolutions per second, and multiplying by gives the angular velocity in .
A rule for the field of a current: grip the wire with the right hand, thumb along the conventional current, and the curled fingers give the direction of the magnetic field lines.
An extended object whose shape and size do not change: the distance between any two of its points stays fixed.
A planned review of the hazards in a method, how likely and serious each one is, and the precautions that reduce the risk.
Rolling in which the contact point has zero velocity relative to the surface, so the centre moves at .
The square root of the mean of the squares of the speeds of the molecules in a gas; the speed in
Motion in which every point of a body moves in a circle about the same line, the axis of rotation.
The state of a body on which the resultant torque about any axis is zero, so its angular velocity does not change.
The kinetic energy a body has because it rotates,
The deflection of alpha particles by the electric repulsion of nuclei, first measured by Geiger and Marsden by firing alpha particles at a thin metal foil; it showed that atoms have a nucleus.