# Unit 2 Definitions ## Unit 2 Definitions

Definitions

A Vector-A vector is any physical quantity that has direction as well as magnitude

A Scalar- A scalar is any physical quantity that has magnitude but is not directional

The moment of a Force - The moment of a force about a point is defined as the force x the perpendicular distance from the line of action of the force to the point

The Principle of moments - The sum of the clockwise moments equals the sum of the anticlockwise moments for a system in equilibrium.

Centre of mass - The centre of mass of a body is the point through which a single force on the body has no turning effect.

A Couple - is a pair of equal and opposite forces acting on a body, but not along the same line.

Torque - The moment of a couple is the torque. It is given by force x perpendicular distance between the lines of action of the force.

Mass - is a measure of an ojects inertia (resistance to change of motion)

Weight - is the force due to gravity acting on an object. It is given by W=mg, where g is the acceleration due to gravity

Displacement - is distance in a given direction

Speed - is defined as change of distance per unit time

Velocity - is defined as change of displacement per unit time

Acceleration - is defined as change of velocity per unit time.

Projectile motion - A projectile is any object acted upon only by the force of gravity. The object moves on a parabolic path.

Projectile like motion - Any form of motion where the object experiences a constant accleration in a different direction to its velocity will be like projectile motion. The object moves on a parabolic path.

Newton's first law of motion - Objects stay at rest or in uniform motion unless acted upon by a resultant force.

Newton's second law - The resultant force is proportional to the acceleration F=ma

Newton's third law - Action and reaction are equal and opposite.

Definitions - Waves

Transverse Waves
- Waves in which the direction of the vibrations is perpendicular to the direction in which the wave travels.

Longitudinal Waves
- Waves in which the direction of the vibrations is parallel to the direction in which the wave travels.

Plane- polarised waves
- are waves in which the vibrations stay in one plane. Only transverse waves can be plane polarised.

Unpolarised waves
- are waves in which the vibrations change from one plane to another.

Displacement - of a vibrating particle is its distance and direction from its equilibrium position.

Amplitude - of a wave is the maximum displacement of a vibrating particle.

Wavelength
- of a wave is the least distance between two particles vibrating in phase (same displacement and velocity at same time)

Time Period - is the time for 1 complete wave to pass a point.

Frequency - is the number of cycles (complete waves) passing a point per second.

Phase difference - between two vibrating particles is the fraction of a cycle between the vibrations of the two particles, measured in degrees or radians.

Progressive wave - Progressive waves travel through a substance (or space if electromagnetic) and transfer energy.

Stationary wave
- A stationary wave is a wave pattern with nodes and antinodes, which are formed when 2 progressive waves of the same frequency and amplitude pass through each other. They do not transfer energy through space.

The principle of superposition
- states that when two waves meet, the total displacement at a point is equal to the sum of the individual displacements at that point.

Node - is a fixed point in a stationary wave where the amplitude is zero.

Antinode - is the fixed point in a stationary wave where the amplitude is a maximum.

Interference - formation of pints of cancellation and reinforcement where two coherent waves pass through each other.

Coherent Waves - are waves with the same frequency and constant phase difference.

Refraction - change of direction of a wave when it crosses a boundary where its speed changes.

Refractive index - speed of light in free space/ speed of light in a substance.

Total internal reflection
- A light ray travelling in a substance is totally internally reflected at a boundary with a substance of lower refractive index, if the angle of incidence is greater than the critical angle.

Critical Angle
- the angle of incidence must exceed the critical angle for total internal reflection to occur.

Optical Fibre
- a thin flexible transparent fibre used to carry light pulses from one end to the other

Multipath dispersion
- the lengthening of a light pulse as it travels along an optical fibre, due to rays that repeatedly undergo total internal reflection having to travel a longer distance than the rays that undergo less total internal reflection.

Spectral dispersion
-this can occur if white light is used in an optical fibre. The speed of light in the glass depends on the wavelength. Violet light travels more slowly than red light. The violet light falls behind the faster red light causing the white light pulse in the optical fibre to become longer.

Monchromatic Light
- this is light of a single wavelength. (a laser is a good example of this - it is highly monchromatic)

Coherent bundle of fibres
(as with medical endoscope)- The fibres at each end are in the same relative positions.

Diffraction - is the spreading out of waves as they pass through a gap or close to an edge

Diffraction Grating
- is a plate with many closely spaced parallel slits.

Maximum Order (diffraction grating) - the maximum order is given by d/wavelength rounded DOWN to the nearest whole number.

Materials Definitions

Density – of a substance is the mass per unit volume

Hooke’s Law – states that the force needed to stretch a spring is directly proportional to the extension of the spring, within the limit of proportionality

Limit of Proportionality – the limit beyond which the extension of a wire or spring is no longer proportional to the force that stretches it.

Spring Constant or stiffness constant – the force per unit extension needed to extend a wire or spring

Tensile Stress –tensile force per unit area

Tensile Strain – ratio of extension to original length

Young’s Modulus – the ratio of tensile stress to tensile strain.

Elastic Limit – the point beyond which a wire is permanently extended (this is plastic deformation)

Ultimate Tensile Stress – this is where a wire loses strength, it is the maximum stress it can withstand before it breaks (sometimes called the breaking stress)

Brittle – A brittle material breaks without noticeable yield – it breaks without deforming

Ductile – A ductile material undergoes plastic deformation before breaking, it can be easily drawn into wires Permissions in this forum:
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