Physics AS

• Quantity


  represented by a number × a unit, (e.g. m = 3.0 kg).
• Scalar

  a quantity that has magnitude only.
• Vector


  a quantity that has magnitude and direction.
• Force


  a push or a pull acting on the body from some external body. Unit: N
• Newton's Third Law


  If a body A exerts a force on a body B, then B exerts an equal and opposite force on A.
• resultant F = m a


  The mass of a body × its acceleration is equal to the vector sum of the forces acting on the body. This vector sum is called the resultant force.
• Density of a material

  =mass/volume, in which mass and volume apply to any sample of the material.. Unit: kg m^-3
• Moment (or torque) of a force about a point


  the force × the perpendicular distance from the point to the line of action of the force, i.e. moment = F × d. Unit: Nm. [N.B. the unit is not J]
• The principle of moments.

  For a system to be in equilibrium, ∑ anticlockwise moments about a point = ∑ clockwise moments about the same point.
• Centre of gravity.


  the single point within a body at which the entire weight of the object may be considered to act
• Displacement


  the shortest distance from A to B, together with the direction. Unit: m.
• Mean Speed


  =total distance travelled/time taken. Unit: ms^-1.
• Instantaneous Speed


  =rate of change of distance. Unit: ms^-1.
• Mean Velocity


  =total displacement/time taken = ∆x/∆t. Unit: ms^-1.
• Instantaneous Velocity


  the rate of change of displacement. Unit: ms^-1
• Mean Acceleration


  =change in velocity/time taken = ∆v/∆t. Unit: ms^-2
• Instantaneous Acceleration


  its rate of change of velocity. Unit: ms^-2
• Terminal Velocity


  the constant, maximum velocity of an object when the resistive forces on it are equal and opposite to the 'accelerating' force (e.g. pull of gravity).
• Work done by a force


  the product of the magnitude of the force and the distance moved in the direction of the force.( W.D. = Fxcos θ ) Unit: J [= Nm]
• Hooke's Law


  The tension in a spring or wire is proportional to its extension from its natural length, provided the extension is not too great.
• Spring constant, k


  the force per unit extension. Unit: Nm-1.
• Energy of a body or system


  the amount of work it can do. Unit: J
• Principle of conservation of energy


  Energy cannot be created or destroyed, only transferred from one form to another. Energy is a scalar.
• Potential energy


  energy possessed by virtue of position. (e.g. Gravitational PE = mgh). Unit: J
• Kinetic energy


  energy possessed by an object by virtue of its motion. Unit: J
• Power


  work done per second, or energy transferred per second. Unit: watt (W) [= Js-1].
• Efficiency of a system

  =100*(useful energy out/total energy in) Unit: none (%)
• Atomic mass number, A [nucleon number]

  the number of nucleons (number of protons + number of neutrons) in its nucleus.
• Lepton


  electrons and electron-neutrinos [and analogous pairs of particles of the so-called second and third generations].
• Hadron


  particles consisting of quarks or antiquarks bound together. Only hadrons (and quarks or antiquarks themselves) can 'feel' the strong force.
• Baryon


  a hadron consisting of 3 quarks or 3 antiquarks. The best known baryons are the nucleons, that is the proton and the neutron.
• Meson


  a hadron consisting of a quark-antiquark pair.
• Black body


  a body (or surface) which absorbs all the electromagnetic radiation that falls upon it. No body is a better emitter of radiation at any wavelength than a black body at the same temperature.
• Wien's displacement law


  The wavelength of peak emission from a black body is inversely proportional to the absolute (kelvin) temperature of the body.
• Absolute or kelvin temperature


  The temperature, T in kelvin (K) is related to the temperature, θ, in celsius (°C) by: T /K= θ /°C + 273.15. At 0 K (-273.15°C) the energy of particles in a body is the lowest it can possibly be.
• Stefan's Law [The Stefan-Boltzmann law]


  The total electromagnetic radiation energy emitted per unit time by a black body is given by power = A σ T4 in which A is the body's surface area and σ is a constant called the Stefan constant. [σ = 5.67 × 10-8 W m-2K-4]
• Luminosity of a star


  the total energy it emits per unit time in the form of electromagnetic radiation. UNIT: W [Thus we could have written luminosity instead of power in Stefan's law (above).]
• vernier calipers


  Vernier calipers are another distance measuring tool that uses a sliding vernier scale
• Electromagnetic force


  neutral hadrons affected, composed of charged quarks
• weak force


  neutrinos present, change in quark flavour, experienced by all leptons and quarks