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DF014PHYSICS

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UNIT 1Physical Quantities

and Measurements1.1 Physical Quantities & Units

1.2 Scalars & Vectors

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1.1

Physical Quantities andUnits

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Learning Outcomes

At the end of this topic, students shouldbe able to:

State basic quantities and their respective SI

units State derived quantities (in terms of basic

quantities) and their respective units andsymbols

Perform conversion between SI units.

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1.1.aPHYSICALQUANTITY Category

Definition

Measured quantity

Basicquantity

Derived

quantity

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Definition:quantity which can be

- observed- measured

2 components:-Basic Quantity-Derived Quantity

PHYSICALQUANTITY

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BASIC QUANTITY

Definition

Quantity Quantitys Symbol

Length l

Mass m

Time t

Temperature T

Electric current I

Amount of substance n

Luminous Intensity Iv

Table 1

which cannot be derived from any

physical quantities.

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DERIVED QUANTITY

Definition

FormulaQuantitys

SymbolDerived Quantity

Fs cosWWork

maFForce

mvpMomentum

m

V

Density

v

t

aAcceleration

eg : (l) (w) (t)VVolume

s

t

vVelocity

Table 2

A physical quantity which can beexpressed in term of base quantity.

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1.1.bUNITS

Category

Definitionstandard size ofmeasurement

of physical quantities.

Base unit

Derived unit

S.I unit

Unitprefixes

Conversionof units

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Definition

standard size of measurement ofphysical quantities.

The common system of units used today :

- S.I unit(System International/metric system)

- cgs unit - UK.

The unit of base quantity is base unit

The unit of derived quantity is derived unit

UNITS

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QuantityQuantitys

SymbolSI Unit

Units

Symbol

Length l meter m

Mass m kilogram kg

Time t second s

Temperature T kelvin K

Electric current I ampere A

Amount of

substancen mole mol

Luminous

IntensityIv candela cd

Base unit

Table 3

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Derived unit

Derived QuantityQuantitys

Symbol

Formula Derived Unit

Velocity v s

t

m s-1

Volume V eg : (l) (w) (t) m3

Acceleration a vt

m s-2

Density m

V

kg m-3

Momentum p mv kg m s-1

Force F ma kg m s-2 @ N

Work W Fs cos kg m2 s-2 @ J

Table 4

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Used for presenting larger and smaller

value.

Example : How to write the readings

below by using unit prefixes?

1. 0.00000000145 m

2. 1234567 g

3. 5000000000 A

4. 1500000000000 J

5. 0.0768921 N

Unit Prefixes

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Prefix Value Symbol

tera x 1012 Tgiga x 109 G

mega x 106 M

kilo x 103 k

deci x 10-1 d

centi x 10-2 c

milli x 10-3 m

micro x 10-6 nano x 10-9 n

pico x 10-12 p

Unit Prefixes

Table 5

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Examples

Change 58000 m to km ????

1 m = 1000 km

58000 m = 58000 (x 10-3 km)

= 58 km

Presents 0.000009 s with unit prefixes.

0.000009 s = 9 x 10-6

s (10-6

= micro = )=9 s

Exercise:

Change a) 1 cm2 to m2b) 360 km h-1to m s-1

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2 2

2 -2 2

2 -4 -2

-1 -1

3

-1

SOLUTIONS :

a) 1 cm ? m

1 (cm) = 1 (10 m)

1 cm = 10 m

b) 360 km h ?m s

360(km) 360(10 m)=

(h) (3600s)

= 100 m s

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S.I Units(system international)

The common system of unit

Use in base and derived unit- see Table 3 and Table 4

Also can be written in equivalent units.- see Table 6

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Physicalquantity S.I Units EquivalentUnitsUnits

Symbol

Pressure kg m-1 s-2 pascal Pa

Force kg m s-2 newton N

Power kg m2 s-3 watt W

Work kg m2 s-2 joule J

Frequency s-1 hertz Hz

**Sometimes when we use certain instruments tomeasure physical quantity, the measurement isnot in SI unit.

Examples for Equivalent Units

Table 5

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Examples of unit conversion :

3

-1 -1

3 -3 3 -9 3

-3

-3 3 -3

-2 3

(10 m)a) 15 km h =15 = 4.17 m s(6060 s)

b) 5 mm = 5(10 m) = 510 m

10 kgc) 7 g cm = 7 = 710 kg m

(10 m)

d) 30 C = 30 + 273 = 303 K