PH1 2015 Flashcards
Quantity
In S.I. a quantity is represented by a number x a unit,
e.g. m = 3.0 kg
Scalar
A scalar is a quantity that has magnitude only
Vector
A vector is a quantity that has magnitude and direction.
Resolving a vector
into components in
particular directions
This means finding vectors (the so-called components)
in these directions, which add together vectorially to
make the original vector, and so, together, are
equivalent to this vector
Density of a
material, ρ
density = mass/volume Unit: kg m^3 or g cm^-3 in which mass and volume apply to any sample of the material.
Moment (or torque) of
a force
The moment (or torque) of a force about a point is defined as the force x the perpendicular distance from the point to the line of action of the force, i.e. moment = F x d Unit: Nm [N.B. the unit is not J]
The principle of
moments
For a system to be in equilibrium, (sum of) anticlockwise
moments about a point = (sum of) clockwise moments about
the same point.
Centre of gravity
The centre of gravity is the single point within a body at
which the entire weight of the body may be considered
to act.
Displacement
The displacement of a point B from a point A is the
shortest distance from A to B, together with the
direction. Unit: m
Mean speed
Mean speed = total distance traveled/ total time taken
ms^-1
Instantaneous speed
Instantaneous speed = rate of change of distance
ms^-1
Mean velocity
mean velocity = total displacement/ total time taken
Instantaneous
velocity
The velocity of a body is the rate of change of
displacement.
ms^-1
Mean acceleration
Mean acceleration = change in velocity/ time taken
ms^-2
Instantaneous
acceleration
The instantaneous acceleration of a body is its rate of
change of velocity.
ms^-2
Terminal velocity
The terminal velocity is 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).
Force, F
A force on a body is a push or a pull acting on the body
from some external body.
N
Newton’s 3rd law
If a body A exerts a force on a body B, then B exerts
an equal and opposite force on A.
(the sum of) F = m a
The mass of a body x its acceleration is equal to the
vector sum of the forces acting on the body. This
vector sum is called the resultant force.
Momentum
The momentum of an object is its mass mass x
its velocity. (p = mv). It is a vector.
UNIT: kg m s-1
or Ns
Newton’s 2nd law
The rate of change of momentum of an object is
proportional to the resultant force acting on it, and
takes place in the direction of that force
The principle of
conservation of
momentum
The vector sum of the momenta of bodies in a system
stays constant even if forces act between the bodies,
provided there is no external resultant force.
Elastic collision
A collision in which there is no change in total kinetic
energy
Inelastic collision
A collision in which kinetic energy is lost.
Work, W
Work done by a force is the product of the magnitude
of the force and the distance moved in the direction of
the force.( W.D. = Fxcos θ )
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, Ep
This is energy possessed by an object by virtue of its
position. Ep = mgh Unit: J
Kinetic energy, Ek
This is the energy possessed by an object when it has
been deformed due to forces acting on it.
Eelastic = ½ Fx or ½ kx^2
Unit: J
Energy
The energy of a body or system is the amount of work
it can do. Unit: J
Power, P
This is the work done per second, or energy
transferred per second. Unit: W [= J s^-1]
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 spring constant is the force per unit extension.
Unit: Nm-1
Stress (looks like wien’s constant)
Stress is the force per unit cross-sectional area when
equal opposing forces act on a body.
Unit Pa or N m
-2
Strain, (squiggly e)
Strain is defined as the extension per unit length due to
an applied stress. Unit: none
Young modulus, E
Young modulus E= Tensilestress/ Tensilestrain
Unless otherwise indicated this is defined for the
Hooke’s law region. Unit: Pa or N m-2
Crystal
Solid in which atoms are arranged in a regular array.
There is a long range order within crystal structures.
Crystalline solid
Solid consisting of a crystal, or of many crystals,
usually arranged randomly. The latter is strictly a
polycrystalline solid. Metals are polycrystalline.
Amorphous solid
A truly amorphous solid would have atoms arranged
quite randomly. Examples are rare. In practice we
include solids such as glass or brick in which there is
no long range order in the way atoms are arranged,
though there may be ordered clusters of atoms.
Polymeric solid
A solid which is made up of chain-like molecules.
Ductile material
A material which can be drawn out into a wire. This
implies that plastic strain occurs under enough stress.
Elastic strain
This is strain that disappears when the stress is
removed, that is the specimen returns to its original
size and shape.
Plastic (or inelastic)
strain
This is strain that decreases only slightly when the
stress is removed. In a metal it arises from the
movement of dislocations within the crystal structure.
Elastic limit
This is the point at which deformation ceases to be
elastic. For a specimen it is usually measured by the
maximum force, and for a material, by the maximum
stress, before the strain ceases to be elastic.
Dislocations in
crystals
Certain faults in crystals which (if there are not too
many) reduce the stress needed for planes of atoms to
slide. The easiest dislocation to picture is an edge
dislocation: the edge of an intrusive, incomplete plane
of atoms.
Grain boundaries
The boundaries between crystals (grains) in a
polycrystalline material.
Ductile fracture
necking
The characteristic fracture process in a ductile
material. The fracture of a rod or wire is preceded by
local thinning which increases the stress.
Brittle material
Material with no region of plastic flow, which, under
tension, fails by brittle fracture.
Brittle fracture
This is the fracture under tension of brittle materials by
means of crack propagation.
Elastic hysteresis
When a material such as rubber is put under stress
and the stress is then relaxed, the stress-strain graphs
for increasing and decreasing stress do not coincide,
but form a loop. This is hysteresis.
Black body
A black body is 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.
landa max = W/T
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 StefanBoltzmann
law]
The total electromagnetic radiation energy emitted per
unit time by a black body is given by power = A σT^4
in
which A is the body’s surface area and σ is a constant
called the Stefan constant. [σ = 5.67 x 10-8 W m^-2 K^-4
Luminosity of a star
The luminosity of a star is 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).]
Intensity
The intensity of radiation at a distance
R from a source
is given by I=P/4piR^2
Lepton
Leptons are electrons and electron -neutrinos [and analogous pairs of particles of the so -called second and third generations].
Hadron
Hadrons are particles consisting of quarks or
antiquarks bound together. Only hadrons (and quarks
or antiquarks themselves) can ‘feel’ the strong force.
Baryon
A baryon is a hadron consisting of 3 quarks or 3
antiquarks. The best known baryons are the nucleons,
i. e. protons and neutrons.
Meson
A meson is a hadron consisting of a quark
-antiquark
pair.
Elastic potential
energy
This is the energy possessed by an object when it has
been deformed due to forces acting on it.
Eelastic = ½ Fx or ½ kx2
Unit: J
