Unit 1 Definitions

Question 1

Quantity

Answer 1

In S.I. a quantity is represented by a number × 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 × 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]

Question 7

The principle of moments

Answer 7

For a system to be in equilibrium, ∑ anticlockwise moments about a point = ∑ 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 travelled/total time taken = Δx/Δt. Unit: ms-1

Question 11

Instantaneous speed

Answer 11

Instantaneous speed = rate of change of distance

Unit: m s-1

Question 12

Mean velocity

Answer 12

Mean velocity = total displacement/total time taken.

Unit: ms-1

Question 13

Instantaneous velocity

Answer 13

The velocity of a body is the rate of change of displacement. Unit: m s-1

Question 14

Mean acceleration

Answer 14

Mean acceleration = change in velocity/time taken = Δv/Δt. Unit: ms-2

Question 15

Instantaneous acceleration

Answer 15

The instantaneous acceleration of a body is its rate of change of velocity.

Unit: m s-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. Unit: 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

Σ F = m a

Answer 19

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.

Question 20

Momentum

Answer 20

The momentum of an object is its mass multiplied by 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 energy possessed by an object by virtue of its motion. Ek = ½mv^2 Unit: J

Question 29

Elastic potential energy

Answer 29

This is the energy possessed by an object when it has been deformed due to forces acting on it. Elastic = ½ Fx or ½ kx2 Unit: J

Question 30

Energy

Answer 30

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

Question 31

Power, P

Answer 31

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

Question 32

Hooke's law

Answer 32

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

Question 33

Spring constant, k

Answer 33

The spring constant is the force per unit extension. Unit: N m-1

Question 34

Stress, σ

Answer 34

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

Question 35

Strain, ε

Answer 35

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

Question 36

Young modulus, E

Answer 36

Young modulus E = tensile stress/tensile strain Unless otherwise indicated this is defined for the Hooke's law region. Unit: Pa or Nm-2

Question 37

Crystal

Answer 37

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

Question 38

Crystalline solid

Answer 38

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

Question 39

Amorphous solid

Answer 39

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 40

Polymeric solid

Answer 40

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

Question 41

Ductile material

Answer 41

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

Question 42

Elastic strain

Answer 42

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

Question 43

Plastic (or inelastic) strain

Answer 43

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 44

Elastic limit

Answer 44

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 45

Dislocations in crystals

Answer 45

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 46

Grain boundaries

Answer 46

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

Question 47

Ductile fracture (necking)

Answer 47

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 48

Brittle material

Answer 48

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

Question 49

Brittle fracture

Answer 49

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

Question 50

Elastic hysteresis

Answer 50

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 51

Black body

Answer 51

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 52

Wien's displacement law

Answer 52

The wavelength of peak emission from a black body is inversely proportional to the absolute (kelvin) temperature of the body. λmax = W/T W = the Wien constant = 2.90 × 10-3 m K

Question 53

Absolute or kelvin temperature

Answer 53

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 54

Stefan's law [The Stefan Boltzmann law]

Answer 54

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 × 10-8 W m-2 K-4

Question 55

Luminosity of a star

Answer 55

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 56

Intensity

Answer 56

The intensity of radiation at a distance R from a source is given by I = P/4 πR ^2 UNIT: W m - 2

Question 57

Lepton

Answer 57

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

Question 58

Hadron

Answer 58

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

Question 59

Baryon

Answer 59

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 60

Meson

Answer 60

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

Question 61

Hysteresis

Answer 61

The work done in contraction is less than the work done in extension. (difference in areas between loading and unloading stress-strain graph).