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Physical Quantities
Definition,Types, Symbols andUnits.
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Definitions of Physical Quantities
A physical quantity is a physical property that can be quantified by measurement.
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Types of physical quantities
• Scalar quantities are quantities that have magnitude only; they
are independent of direction. (time, temperature, mass, density, energy,…)• Vector quantities are quantities that have magnitude and
direction. (displacement, velocity, moment, force, ….)
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Symbols for physical quantities
Usually, the symbols for physical quantities are chosen to be a single letter of the Latin or Greek alphabet, and are often printed in italic type (F, t, m, ……). Often, the symbols are modified by subscripts and superscripts, to specify what they refer to - for instance Ek is usually used to denote kinetic energy and cp heat capacity at constant pressure.
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Physical Quantities
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Vector presentation
• When vectors are written, they are represented by a single letter in bold type or with an arrow above the letter, such as or . Some examples of vectors are displacement (e.g. 120 cm at 30°) and velocity (e.g. 12 meters per second north). The only basic SI unit that is a vector is the meter. All others are scalars. Derived quantities can be vector or scalar, but every vector quantity must involve meters in its definition and unit.
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Vector notation
• Vectors are distinguished from scalars by writing them in special ways. A widely used convention is to denote a vector quantity in bold type, such as A, and that is the convention that will be used. you may also encounter the notation Ū or ḛ.
• The magnitude of a vector A is written as |A|.
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Representation of vectors in Cartesian coordinates
The vector can be represented by its components (magnitude)in (x,y,z) directions. The direction of the vector can be represented by a unit vector u = i + j + k,
so: v = vx i + vy j + vz k
where the units can be anything
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Representation of vectors in Cartesian coordinates (cont.)
• F = Fx i + Fy j + k
Where:Fx = F cos α
Fy = F cos β
Fz = F cos γ
α, β and γ are the angles the force vector (F) making with x, y and z axis respectively. And
F = √ Fx2 + Fy
2 + Fz2
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Vector Mathematical Operation(adding, subtracting, multiplying ..) • you have to consider both the magnitude and the direction.
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Vectors Addition
Adding two vectors in Cartesian form v1 and v2
v3 = v1 + v2
v3 = (vx1 i + vy1 j + vz1 k) +(vx2 i + vy2 j + vz2 k)
v3 = (vx1 + vx2) i + (vy1 + vy2) j + (vz1 + vz2) k
That is, the components of a sum are the sums of the components.
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Units of physical quantities
• Most physical quantities Q include a unit. Neither the name of a physical quantity, nor the symbol used to denote it, implies a particular choice of unit.
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Systems of Units
• The Imperial units or the Imperial system is a collection of English units, first defined in the Weights and Measures Act of 1824, later refined (until 1959) and reduced. The units were introduced in the United Kingdom and its colonies, including Commonwealth countries, but excluding the then already independent United States.
• The United States customary system (also called American system or, more rarely, "English units") is the most commonly used system of measurement in the United States. It is similar but not identical to the British Imperial units.
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Systems of Units (cont.)
• The most widely used system of units and measures around the world is the Systeme International d'Unites (SI), the modern form of the metric system. This originated in France, where in 1790 the French Academy of Science was commissioned to design a new system of units (the International System of Unit).
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International system of units (SI)• Basic units:
Physical quantity SI unit Unit symbol
Length meter m
Mass kilogram kg
Time second s
Electric current ampere A
Degree of temperature kelvin K
Amount of substance mole mole
Light (luminous) intensity candela cd
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International system of units (SI) (cont.)
• Associated with basic units are a variety of supplementary derived units.
• Derived units:Derived quantity SI unit Unit symbol
Area square meter m2
Volume cubic meter m3
Velocity meter per second m/s
Acceleration meter per second square m/s2
Density Kilogram per cubic meter Kg/m3
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International system of units (SI) (cont.)• Derived units with special names and symbol:
Physical quantity SI unit Unit symbol
Force, Weight newton N = kg m/s2
Work, Energy, Quantity of Heat joule J = N m
Power watt W = N m/s = J/s
Pressure, Stress pascal Pa = N/m2
Electric charge coulomb C = A s
Electric potential volt V = W/A
Electric capacitance frad F = A s/V
Electric resistance ohm Ω = V/A
Frequency hertz Hz = s-1
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International system of units (SI) (cont.)
• In the International system of unit, a unit is chosen for a particular purpose and larger and smaller are obtained by applying a prefix to this unit and multiplying or dividing by 10 or power of 10.
• Larger units, obtained by multiplying by 1000, etc., are called ‘multiples’, smaller units, obtained by multiplying by 0.1, etc. are termed ‘sub-muliplies.
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International system of units (SI) (cont.)• Multiplying factors:
Factor Prefix Symbol
1 000 000 000 [109] giga G
1 000 000 [106] mega M
1 000 [103] kilo k
100 [102] hecto h
10 [10] deca da
0.1 [10-1] deci d
0.01 [10-2] centi c
0.001 [10-3] mili m
0.000 001 [10-6] micro μ
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International system of units (SI) (cont.)• Units outside the SI that accepted for use with the SI:
Derived quantity Unit symbol Value
Minute min 1 min = 60 s
Hour h 1 h = 60 min = 60 s
Day day 1 day = 24 h = 86400 s
Degree angle o 1o = (π/180) rad
Liter l, (L) 1 L = 1 dm3 = 1000 cm3 = 0.001 m3
Metric ton (tonne) t 1 t = 1000 kg
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International system of units (SI) (cont.)
• Other units outside the SI that are currently accepted for use with the SI:
Derived quantity Unit symbol Value
Nautical mile Nautical mile 1 nautical mile = 1852 m
Knot Knot 1 knot = 1.852 km/h
Hectare ha 1 ha = 1 hm2 = 104 m2
Bar bar 1 bar = 0.1 MPa = 100 kPa = 105 Pa
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SI unit rules and style conventions• Unit symbols are placed after the numerical value, leaving a
space between the value and symbol. e.g. 5 V not 5V.• Only one prefix can be applied to a unit at a time. e.g. 1000
kilonewton must not express 1kilokilonewton but as 1 meganewton, (1000 kN ≠ 1 kkN = 1 MN).
• The correct use of upper and lower case letters (capital and small letters) is important. i.e. m- meter; but M- mega; k- kilo; K- kelvin.
• Symbols must not made plural by adding ‘s’ since ‘s’ is symbol of second. e.g. 10 kg not 10 kgs.
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SI unit rules and style conventions (cont.)
• When a prefix is attached to a unit there should be no space between the prefix and the unit; i.e. mm- millimeter, kW- kilowatt… etc.
• When a complex unit is formed by multiplying two units together, however, the symbol should be separated by a space; e.g. N m- newton meter (the unit of torque).
• Abbreviations such as sec, cc, or mps are avoided and only standard unit symbols, prefix symbols, unit names, and prefix names are used (s, cm3, m/s).
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SI unit rules and style conventions (cont.)
• Unit symbols are generally written in lower case letters, except when the name of the unit is derived from a proper name. (Note that the name of a unit which is derived from a proper name is written out in full, such as ampere or hertz, the name is not capitalized. The only exception to this is Celsius).
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Dimensional analysis
• Technique used in the physical and engineering to reduce physical properties such as acceleration, velocity, energy and others to their fundamental dimensions of length(L), mass (M), and time (T). This technique facilitate the study of interrelationships of systems (or models of systems) and their properties. Acceleration, for example, is expressed as length per unit of time squared (LT-2); whether the units of length in the English or the metric system is immaterial.
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Physical quantities: Time
Time (t)The fundamental unit of time suggested by SI system isthe second, since 1967 defined as the second ofinternational Atomic Time, based on the radiationemitted by a Caesium-133 atom in the ground state. Based on the second as base unit, the following unitsare in use:• minute (1 min) = 60 s• hour (1h) = 60 min = 3.6 ks• Julian day (1 day) = 24 h = 86.4 ks
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Physical quantities: Displacement, Area & Volume
Displacement and distance (L)Displacement is measured as meters (m), or sometimes moreconveniently as kilometers, millimeters or centimeters. Distance(scalar) is the magnitude of the displacement (vector).Area (A)Any flat, curved, or irregular expanse of a surface. It is measuredin meter square (m2). Volume (V)The volume of any solid, liquid, gas, plasma, or vacuum is howmuch three-dimensional space it occupies, often quantifiednumerically. It is measured in cubic meter (m3) or liter (l). • 1 l = 1000 cm3 = 0.001 m3
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Physical quantities: Angle
Angle (θ)• Angle is the figure formed by two rays sharing
a common endpoint, called the vertex of the angle. The magnitude of the angle is the "amount of rotation" that separates the two rays, and can be measured by considering the length of circular arc swept out when one ray is rotated about the vertex to coincide with the other.
• The degree and the radian are by far the most common. The turn (or full circle, revolution, rotation, or cycle) is one full circle.
• 1 turn = 360° = 2π (rad)
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Physical quantities: Linear & Angular Velocities
Velocity (v) and speedVelocity is measured as meters per second (m/s), or sometimes moreconveniently as kilometers per hour (km/h). Useful conversions are m/s = 3.6 km/h, and 0.278 m/s = 1 km/h). Thespeed is the magnitude of the velocity vector. Angular velocity (ω)Angular velocity is a measure of the angular displacement per unittime.The angular velocity can be measured in revolution per minute (rpm). ω = 2 π N/60• Where:
N = revolution per minute (rpm)
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Physical quantities: Angular & Linear Velocity
Angular and linear velocityThe linear velocity of a particle is related toangular velocity by :
v = ω r v = (2 π N/60) r
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Physical quantities: Linear AccelerationAcceleration (a)Acceleration or deceleration is the rate of change of speed. It is measured as metersper second per second or m/s2. If the speed increases from u m/s (initial velocity) to vm/s (final velocity) during t seconds (time), then the average acceleration a m/s2 isgiven by a = (v-u)/t m/s2
Acceleration due to gravity (g) In physics, gravitational acceleration is the acceleration on an object caused bygravity, a conventional standard value of exactly 9.80665 m/s2 (g = 9.81 m/s2)
* Car acceleration and deceleration indicationAnother way to indicate the car acceleration is the time taken by the car form 0velocity to reach 100 km/h, or (0- 60 mph). * A convenient way to measure braking action is to equate (compare) vehicle
deceleration to the gravity acceleration constant [g]. Example: declaration = 0.3 g.
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Physical quantities: Angular acceleration
Angular acceleration (α)Angular acceleration is the rate of change of angularvelocity over time. In SI units, it is measured in radiansper second squared (rad/s2).
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Physical quantities: Mass & Weight
Mass (m) and weight (w)A 'body' contains a certain amount of stuff ormatter called mass (m). The unit of mass is thekilogram (kg).The pull of earth -the force ofgravity- acting on this mass is the weight (w)ofthe body. The unit of weight is the newton (N). w = mg Where: g is the acceleration of falling body dueto gravity in meter per second square (m/s2).
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Physical quantities: Mass moment of inertia
Mass moment of inertia (I): In classical mechanics, moment of inertia, also called
mass moment of inertia, rotational inertia, or the angular mass, (SI units kg m2), is a measures of an object’s resistance to changes to its rotation.
I = ∫ r2 dm
Where:
m is the mass,
r is the perpendicular distance to the axis of rotation.
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Physical quantities: Density
Density (ρ)Density is the mass of a substance per unit volume (kg/m3). The density of water is, for practical purposes, 1000 kg/m3 or 1 kg/l. (The litre (l) is 10-3 m3.)
• The density of solids and liquids are usually stated in g/cm3, while gases are usually in kg/m3.
Relative density or specific gravity• relative density of the substance = (mass of a substance / mass of an
equal volume of water)• This ratio is called the relative density of the substance, and
represents how many times it is heavier or lighter than the same volume of water. Note that relative density has no units. Example of substances relative densities (oxygen = 0.0014, steel=8.0, lead =11.4, mercury = 13.6).
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Physical quantities: Force
Force (F)The force is a measurable influence tending to causemovement of body (its intensity). The unit of force is newton,1 newton is the value of a force which if exerted upon a mass(m) of one kilogram gives it an acceleration (a) of 1 m/s2. F = m a, 1 N = (1 kg) (1 m/s2) = 1 kg m/s2
There are different types of forces; external force, internalforce, friction force, inertia force and reaction force.
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Physical quantities: Pressure Pressure (p)
Pressure is the force per unit area; the unit is N/m2 or the Pascal (Pa). Larger practical units are kN/m2 (kPa) and MN/m2 (MPa). Note that
1 MN/m2 (MPa) = 1N/mm2 A pressure of 7 MPa means that each mm2 subject to the pressure has a force of 7 N
acting on it, and the total force on the surface will be the product of the pressure and the area.
Atmospheric pressureAir has weight. The atmosphere above the earth produces a pressure at sea level of approximately 1 bar, where 1 bar = 105 N/m2 or 105 Pa. Standard atmospheric pressure (atm) is 1.01325 bar.
1 atm = 760 mm Hg (Torr) (mercury column) = 10333 mm H2O (water column)
Gauge and absolute pressureThe ordinary pressure gauge gives readings measured above atmospheric pressure. To obtain the absolute pressure, that is the pressure measured above a perfect vacuum, atmospheric pressure must be added to the gauge reading:
absolute pressure = gauge pressure + atmospheric pressure
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Physical quantities: Torque
Torque (T)When a force (F) acts on a body pivoted on a fixed axis, the product of the force perpendicular to the radius, and the radius at (r) which it acts, is termed the turning moment of the force or torque. Torque (T) is measured in newton meters (N m).
T = F r
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Physical quantities: Work & PowerWork (W)Work is done when a force overcomes resistance and causes movement. Work ismeasured by the product of the force (f) and the distance moved (s) in the direction ofthe force, the unit being the joule (J): W = F s If the force causes no movement, then no work is done, or if the force in the appositedirection of the movement, then the work is negative. Power (P)Power is the rate of doing work. The unit, the watt, is a rate of working of 1 joule persecond (1 J/s) power = work done per second = [W / t] {N m /s}, {J/s}, (W)The relation between power and torque: If a tangential force is applied to a shaft, then the work done per one revolution is: W = F s = F (2 π r) = (2 π) TSince the power is P = W/t = (2 π / t) T, then: P = ωT, P = (2 π N/60) T Using these formulae, the power can be calculated from the torque and speed of ashaft.
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Physical quantities: Temperature
Temperature (t) Temperature is a physical property of matter that
quantitatively expresses the common notions of hot and cold. Quantitatively, temperature is measured with thermometers.The International System of Units (SI) defines a scale and unit for the thermodynamic temperature by using the kelvin temperature. The unit symbol of the kelvin is K. While the Kelvin scale is the principal temperature scale for use in science and engineering, much of the world uses the Celsius scale (°C) for most temperature measurements.
1 K = 1°C = 1.8 °F = 1.8 °R
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Physical quantities: Energy
Energy (E)In all such energy transformation processes, the total energy remains the same. Energy may not be created nor destroyed.
• Any form of energy can be transformed into another form. When energy is in a form other than heat, it may be transformed with good or even perfect efficiency, to any other type of energy.
• Measurement of energy in the SI unit is the joule. In addition to the joule, other units of energy include the kilowatt hour (kW h) and the British thermal unit (Btu). These are both larger units of energy. One kW h is equivalent to exactly 3.6 million joules, and one Btu is equivalent to about 1055 joules.
• There are different types of energies; kinetic, potential, mechanical, thermal, chemical, electric and nuclear energy.
