PH1 2015

Question 1

Quantity

Answer 1

In S.I. a quantity is represented by a number x a unit,

e.g. m = 3.0 kg

Question 2

Scalar

Answer 2

A scalar is a quantity that has magnitude only

Question 3

Vector

Answer 3

A vector is a quantity that has magnitude and direction.

Question 4

Resolving a vector
into components in
particular directions

Answer 4

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

Question 5

Density of a

material, ρ

Answer 5

density = mass/volume
Unit: kg m^3
 or g cm^-3
in which mass and volume apply to any sample of the
material.

Question 6

Moment (or torque) of

a force

Answer 6

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]

Question 7

The principle of

moments

Answer 7

For a system to be in equilibrium, (sum of) anticlockwise
moments about a point = (sum of) clockwise moments about
the same point.

Question 8

Centre of gravity

Answer 8

The centre of gravity is the single point within a body at
which the entire weight of the body may be considered
to act.

Question 9

Displacement

Answer 9

The displacement of a point B from a point A is the
shortest distance from A to B, together with the
direction. Unit: m

Question 10

Mean speed

Answer 10

Mean speed = total distance traveled/ total time taken

ms^-1

Question 11

Instantaneous speed

Answer 11

Instantaneous speed = rate of change of distance

ms^-1

Question 12

Mean velocity

Answer 12

mean velocity = total displacement/ total time taken

Question 13

Instantaneous

velocity

Answer 13

The velocity of a body is the rate of change of
displacement.
ms^-1

Question 14

Mean acceleration

Answer 14

Mean acceleration = change in velocity/ time taken

ms^-2

Question 15

Instantaneous

acceleration

Answer 15

The instantaneous acceleration of a body is its rate of
change of velocity.
ms^-2

Question 16

Terminal velocity

Answer 16

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).

Question 17

Force, F

Answer 17

A force on a body is a push or a pull acting on the body
from some external body.
N

Question 18

Newton’s 3rd law

Answer 18

If a body A exerts a force on a body B, then B exerts

an equal and opposite force on A.

Question 19

(the sum of) F = m a

Answer 19

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.

Question 20

Momentum

Answer 20

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

Question 21

Newton’s 2nd law

Answer 21

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

Question 22

The principle of
conservation of
momentum

Answer 22

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.

Question 23

Elastic collision

Answer 23

A collision in which there is no change in total kinetic

energy

Question 24

Inelastic collision

Answer 24

A collision in which kinetic energy is lost.

Question 25

Work, W

Answer 25

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

Question 26

Principle of conservation of energy

Answer 26

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

Question 27

Potential energy, Ep

Answer 27

This is energy possessed by an object by virtue of its | position. Ep = mgh Unit: J

Question 28

Kinetic energy, Ek

Answer 28

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

Question 29

Energy

Answer 29

The energy of a body or system is the amount of work | it can do. Unit: J

Question 30

Power, P

Answer 30

This is the work done per second, or energy | transferred per second. Unit: W [= J s^-1]

Question 31

Hooke’s law

Answer 31

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

Question 32

Spring constant, k

Answer 32

The spring constant is the force per unit extension. | Unit: Nm-1

Question 33

Stress (looks like wien's constant)

Answer 33

Stress is the force per unit cross-sectional area when equal opposing forces act on a body. Unit Pa or N m -2

Question 34

Strain, (squiggly e)

Answer 34

Strain is defined as the extension per unit length due to | an applied stress. Unit: none

Question 35

Young modulus, E

Answer 35

Young modulus E= Tensilestress/ Tensilestrain Unless otherwise indicated this is defined for the Hooke’s law region. Unit: Pa or N m-2

Question 36

Crystal

Answer 36

Solid in which atoms are arranged in a regular array. | There is a long range order within crystal structures.

Question 37

Crystalline solid

Answer 37

Solid consisting of a crystal, or of many crystals, usually arranged randomly. The latter is strictly a polycrystalline solid. Metals are polycrystalline.

Question 38

Amorphous solid

Answer 38

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.

Question 39

Polymeric solid

Answer 39

A solid which is made up of chain-like molecules.

Question 40

Ductile material

Answer 40

A material which can be drawn out into a wire. This | implies that plastic strain occurs under enough stress.

Question 41

Elastic strain

Answer 41

This is strain that disappears when the stress is removed, that is the specimen returns to its original size and shape.

Question 42

Plastic (or inelastic) | strain

Answer 42

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.

Question 43

Elastic limit

Answer 43

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.

Question 44

Dislocations in | crystals

Answer 44

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.

Question 45

Grain boundaries

Answer 45

The boundaries between crystals (grains) in a | polycrystalline material.

Question 46

Ductile fracture | necking

Answer 46

The characteristic fracture process in a ductile material. The fracture of a rod or wire is preceded by local thinning which increases the stress.

Question 47

Brittle material

Answer 47

Material with no region of plastic flow, which, under | tension, fails by brittle fracture.

Question 48

Brittle fracture

Answer 48

This is the fracture under tension of brittle materials by | means of crack propagation.

Question 49

Elastic hysteresis

Answer 49

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.

Question 50

Black body

Answer 50

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.

Question 51

Wien’s displacement | law

Answer 51

The wavelength of peak emission from a black body is inversely proportional to the absolute (kelvin) temperature of the body. landa max = W/T

Question 52

Absolute or kelvin | temperature

Answer 52

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.

Question 53

Stefan’s law [The StefanBoltzmann law]

Answer 53

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

Question 54

Luminosity of a star

Answer 54

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).]

Question 55

Intensity

Answer 55

The intensity of radiation at a distance R from a source is given by I=P/4piR^2

Question 56

Lepton

Answer 56

``` Leptons are electrons and electron -neutrinos [and analogous pairs of particles of the so -called second and third generations]. ```

Question 57

Hadron

Answer 57

Hadrons are particles consisting of quarks or antiquarks bound together. Only hadrons (and quarks or antiquarks themselves) can ‘feel’ the strong force.

Question 58

Baryon

Answer 58

A baryon is a hadron consisting of 3 quarks or 3 antiquarks. The best known baryons are the nucleons, i. e. protons and neutrons.

Question 59

Meson

Answer 59

A meson is a hadron consisting of a quark -antiquark pair.

Question 60

Elastic potential | energy

Answer 60

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