Physics Definitions
Motion
Speed: distance travelled per unit time
Velocity: change in displacement per unit time
Acceleration: change in velocity per unit time
Thinking distance: distance travelled during
reaction time of driver before brakes are applied
Braking distance: distance traveled while the
brakes are applied before the vehicle is brought to
rest
Stopping distance: total distance travelled in the
time it takes to stop a vehicle, thinking distance +
braking distance
Mass and Weight
Mass: measure of the quantity of matter in an object
at rest relative to the observer
Weight: the gravitational force on an object that has
mass
Gravitational field strength: force per unit mass
Centre of mass/gravity: the point on an object
where all the mass/weight seems to be concentrated
and where,
the point at which all the weight acts on an object
Density
Density: mass per unit volume
Force
Resultant force: change in momentum per unit time
Friction: a force that resists motion and produces
heating
Centripetal force: force acting towards the centre of
motion, perpendicular to the direction of motion,
which keeps a body moving in a circular path
Hooke’s law: the extension of a spring is
proportional to the force applied until the limit of
proportionality
Spring constant: force per unit extension
Limit of proportionality: the point after which
load-extension graph becomes non-linear,
the point after which a spring no longer follows
Hooke’s law,
the point after which the extension of a spring is no
longer proportional to force
Moment: force x perpendicular distance from pivot,
it is the measure of the turning effect of the force
Principle of moments: for an object in equilibrium,
the sum of clockwise moments about any point is
equal to the sum of anticlockwise moments about
the same point
Momentum
Momentum: mass x velocity
Impulse: force x time for which force acts
Conservation of momentum: the sum of the initial
momentum is equal to the sum of the final
momentum of objects in a collision given that no
external forces act on the system
Energy
Work done: force x distance moved in the direction
of the force
Conservation of energy: energy cannot be created
or destroyed, it can only be transferred from one
form to another
Renewable: a resource that is replaced at the same
rate as it is used up so it does not run out
Efficiency:
●​ratio of useful energy output to total energy input
●​ratio of useful power output to total power input
Power: work done per unit time,
energy transferred per unit time
Pressure
Pressure: force per unit area
Thermal Physics
Absolute zero: lowest possible temperature where
particles have least kinetic energy (0 kelvin, -273 °C)
Specific heat capacity: energy required to increase
the temperature of a unit mass by a unit temperature
(by 1°C)
Specific latent heat: energy required to change the
state of a unit mass without a change in temperature
Melting point: temperature at which a substance
change from solid to liquid
Boiling point: temperature at which a substance
changes from liquid to gas
Freezing point: temperature at which a substance
changes from a liquid to a solid
Condensing point: temperature at which a
substance changes from a gas to a liquid
Waves (general)
Wave motion: propagation of a disturbance with
transfer of energy without the transfer of matter
Transverse wave: wave in which particles oscillate
perpendicular to the direction of energy transfer, it
consists of peaks and troughs
Longitudinal wave: wave in which particles
oscillate parallel to direction of energy transfer, it
consists of compressions and rarefactions
Wavefront: an imaginary line that connects all the
points in a wave that are vibrating in phase
Compression: region in a longitudinal wave where
particles are closer together than normal and the
pressure is higher than normal
Rarefaction: region in a longitudinal wave where
particles are further apart than normal and the
pressure is lower than normal
Time period: the time taken for one wavelength to
pass a point
Frequency: number of wavelengths that pass a
point per unit time
Wavelength: distance between two consecutive,
identical points on a wave
Amplitude: maximum distance from the mean
position
Light
Normal: line which is perpendicular to a surface
Angle of incidence: angle between incident ray and
the normal to a surface
Angle of reflection: angle between reflected ray
and the normal to a surface
Angle of refraction: angle between refracted ray
and the normal to a surface
Refractive index: sin(i)/sin(r), where i is the angle of
incidence and r is the angle of refraction
Critical angle: the angle of incidence which
produces an angle of refraction of 90° given that the
light ray is travelling from a denser medium to a less
dense medium
Total internal reflection: when a light ray strikes
the inner boundary of an object and all of the ray is
reflected back inside it, without any rays being
refracted, given that the light ray is travelling rom a
denser medium to a less dense medium
Lens
Focal length: distance between the optical centre of
a lens and its focal point/principal focus.
Principal axis: line through the optical center of a
lens at right angles to the lens
Focal point / Principal focus: the point on the
principal axis of a lens to which light rays parallel to
the principal axis converge, or appear to diverge
from
Linear magnification: ratio of image length to
object length
Sound
Ultrasound: sound wave with a frequency higher
than 20,000 Hz
Electricity​
Current: number of charges passing a point per unit
time
E.M.F. (electromotive force): the electrical work
done by a source in moving a unit charge around a
complete circuit
P.D. (potential difference): the work done by a unit
charge passing through a component
Ohm’s law: the current in a resistor is proportional
to the voltage across it given that temperature
remains constant
Kilowatt-hour: energy used by a 1 kilowatt electrical
appliance in 1 hour
Live wire: the wire that supplies current and e.m.f
from power stations to houses
Neutral wire: wire that provides a complete circuit
with the live wire
Nuclear Physics
Proton/Atomic number: the number of protons in
the nucleus of an atom
Nucleon/Mass number: the total number of protons
and neutrons in the nucleus of an atom
Radioactive decay: change in an unstable nucleus
that can result in the emission of α-particles
or β-particles and/or γ-radiation, these changes are
spontaneous and random
α-particle: helium nucleus, consists of 2 protons
and 2 neutrons
β-particles: high speed electron from the nucleus
γ-radiation: high frequency electromagnetic waves
Fusion: joining of smaller nuclei to form a larger
nuclei with the release of energy
Fission: splitting of a larger nucleus when it absorbs
neutrons to smaller daughter nuclei with the release
of energy and two or more neutrons
Half-life: the time taken for half the nuclei of that
isotope in any sample to decay
Space Physics
Average orbital speed: 2πr/T, where r is the
average radius of the orbit and T is the orbital period
Redshift: increase in the observed wavelength and
decrease in the observed frequency of
electromagnetic radiation emitted from distant,
receding stars and galaxies