# Physics A2-level Definitions

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Newton's first law
When the resultant force on an object is 0, it either stays at rest or travels in a straight line at a constant velocity.
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Newton's second law
The resultant force acting on an object is equal to the rate of change of momentum of the object. The resultant force and change in momentum are in the same direction.
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Newton's third law
When one body exerts a force on another the other body exerts an equal and opposite force on the first body.
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Linear momentum
The product of an object's mass and its velocity. Momentum is a vector quantity.
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Net force
An object's rate of change of momentum.
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Principle of conservation of momentum
In a closed system, when bodies interact, the total momentum in any specified direction remains constant.
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Perfectly elastic collision
A collision in which kinetic energy is conserved.
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Inelastic collision
A collision in which kinetic energy is not conserved; some is transferred to other forms of energy such as heat.
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Gravitational field strength
Force per unit mass.
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Newton's law of gravitation
Any two point masses attract each other with a force that is directly proportional to the product of their masses and inversely proportional to the square of their separation.
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Period (circular motion)
The time taken for an object to complete one orbit.
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Geostationary orbit
The orbit of an artificial satellite which has a period equal to one day so that the satellite remains above the same point on the Earth's equator.
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Displacement
The distance of an oscillating particle from its equilibrium position.
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Amplitude
The maximum displacement of a particle from its equilibrium position.
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Period (oscillations)
The time taken for one complete oscillation of a vibrating object.
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Frequency
The number of oscillations of a particle per unit time.
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Angular frequency
The product of 2 x pi x frequency or (2 x pi)/period
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Phase difference
The fraction of an oscillation between the vibrations of two oscillating particles.
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Simple harmonic motion
Motion of an oscillator where its acceleration is directly proportional to its displacement from its equilibrium position and is directed towards that position.
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Pressure
Force per unit area.
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Internal energy
The sum of the random distribution of kinetic and potential energies associated with the molecules of a system.
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Specific heat capacity
The energy required per unit mass of a substance to raise its temperature by 1K.
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Latent heat of fusion
The energy required to change the state of a fixed amount of a substance from solid to liquid without any change in temperature at a given pressure.
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Latent heat of vapourisation
The energy required to change the state of a fixed amount of a substance from liquid to gas without any change in temperature at a given pressure.
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Boyle's law
The pressure exerted by a fixed amount of gas is inversely proportional to its volume provided the temperature of the gas remains constant.
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Electric field strength
Force per unit positive charge.
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Tesla
The magnetic flux density when a conductor perpendicular to the field of length 1m and with a current of 1A passing through it experiences a force of 1N.
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Magnetic flux
The product of magnetic flux density normal to a circuit and the cross-sectional area of the circuit.
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The product of magnetic flux and the number of turns.
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Weber
The magnetic flux when a circuit of cross-sectional area 1m^2 is normal to a magnetic field with magnetic flux density 1T.
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The magnitude of the induced e.m.f. is proportional to the rate of change of magnetic flux linkage.
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Lenz's law
The induced current or e.m.f. is in a direction so as to produce effects which oppose the change producing it.
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Capacitance
The ratio of charge stored by a capacitor to the potential difference across it.
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The capacitance of a capacitor that stores 1C of charge when the potential difference across it is 1V.
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Impulse
The product of a force and the time for which it acts.
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The angle subtended at the centre of a circle by an arc equal in length to the radius.
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Time constant
The time taken for the current, charged stored or p.d. to fall to 1/e of its original value when a capacitor discharges through a resistor.
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Centripetal acceleration
The acceleration of an object moving in a circle; it is always directed towards the centre of the circle.
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Centripetal force
The net force acting on an object moving in a circle; it is always directed towards the centre of the circle.
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Resonance
The forced motion of an oscillator characterised by maximum amplitude when the forcing frequency matches the oscillator's natural frequency. The system absorbs maximum energy.
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Proton number (Z)
The number of protons in the nucleus of an atom.
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Nucleon number (A)
The number of neutrons and protons in the nucleus of an atom.
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Isotopes
Nuclei of the same element with a different number of neutrons but the same number of protons.
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Activity
The rate of decay of nuclei in a radioactive sample.
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Decay Constant
The probability of an isotope decaying per unit time interval.
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Half-life
The mean time taken for half the number of active nuclei in a radioactive sample to decay.
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Binding energy
The minimum external energy required to seperate all the neutrons and protons of a nucleus.
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Binding energy per nucleon
The ratio of the minimum external energy required to seperate all the neutrons and protons of a nucleus to the total number of neutrons and protons.
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Intensity
Power per unit cross-sectional area.
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Particles that consist of quarks and hence are affected by the strong nuclear force.
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Leptons
Particles that are unaffected by the strong nuclear force.
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Up quark (u)
Q: +2/3; B: +1/3; S: 0
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Down quark (d)
Q: -1/3; B: +1/3; S: 0
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Strange quark (s)
Q: -1/3; B: +1/3; S: -1
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Activity
The rate of nuclei in a radioactive sample.
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Decay constant
The probability of an isotope decaying per unit time interval.
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Half-life
The mean time taken for half the number of active nuclei in a radioactive sample to decay.
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Binding energy
The minimum external energy required to seperate all the neutrons and protons of a nucleus.
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Binding energy per nucleon
The minimum external energy required to seperate all the neutrons and protons of a nucleus to the total number of protons and neutrons.
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Intensity
Power per unit cross-sectional area.
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Doppler effect
The change in frequency and wavelength of a wave caused by the relative movement between source and detector.
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Astronomical unit
The average distance of the Earth from the Sun.
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Parsec
The distance that gives a parallax angle of 1 arc second.
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Light-year
The distance travelled by light in one year.
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For an infinite, uniform and static universe, the night sky should be bright because of light received in all directions from stars.
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Cosmological principle
The universe has the same large-scale structure when observed from any point in the universe.
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Critical density
The density of the universe that will give rise to a flat universe.
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## Other cards in this set

### Card 2

#### Front

The resultant force acting on an object is equal to the rate of change of momentum of the object. The resultant force and change in momentum are in the same direction.

#### Back

Newton's second law

### Card 3

#### Front

When one body exerts a force on another the other body exerts an equal and opposite force on the first body.

### Card 4

#### Front

The product of an object's mass and its velocity. Momentum is a vector quantity.

### Card 5

#### Front

An object's rate of change of momentum.