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AS PHYSICS – MODULE 2: FOUNDATIONS OF PHYSICS
2.1 Physical quantities and units
2.1.1 Physical quantities
• Physical quantity – property of an object/phenomenon that can be measured
• Some quantities are just numbers e.g. efficiency so just have a numerical value
but most physical quantities have a numerical value and a unit
• Estimates of common physical quantities:
Length of ruler = 30 cm Walking speed = 1 ms-1
Height of a person = 1.5 m Speed of car on a motorway = 30 ms-1
Mass of an apple = 0.1 kg Acceleration of free fall = 10 ms-2
Mass of a person = 70 kg Density of air = 1 kgm-3
Mass of a car = 1000 kg Density of water = 1000 kgm-3
2.1.2 S.I units
• Some quantities have many different units e.g. length has metres, inches and
miles
• Therefore, scientists use a standard system of units called the Systѐme
International d’Unitѐs (International System of Units)
SI base units:
Quantity Base unit Unit symbol
length metre m
mass kilogram kg
time second s
electric current ampѐre A
temperature kelvin K
amount of substance mole mol
• Unit symbol is written in lower case unless unit is named after a person
Derived units:
• Derived units without special unit names:
Derived quantity Derived unit
area m2
volume m3
speed ms-1
acceleration ms-2
density kgm-3
momentum kgms-1
1|T. Chaudhary
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• Derived units with special unit names:
Derived quantity Unit name Unit symbol In other SI units
force newton N kgms-2
pressure pascal Pa Nm-2
energy or work done joule J Nm
power watt W Js-1
electrical potential difference volt V JC-1
electric resistance ohm Ω VA-1
electric charge coulomb C As
frequency hertz Hz s-1
Checking the homogeneity of physical equations using S.I base units:
• If an equation is correct, it must have same units for each quantity (i.e. they must
be homogeneous)
• Base units can be used to check whether an equation representing physical
quantities could be correct
• EG: v = u + at
• Left side: ms-1
• Right side: ms-1 + (ms-2 x s) → ms-2 x s = ms-1 ∴ equation is homogeneous
Prefixes:
• SI uses prefixes to show multiples and fractions of units
• Prefixes for multiples all have initial capitals (apart from kilo)
• Prefixes for fractions are all lower case
Prefix name Prefix symbol Factor
peta P 1015
tera T 1012
giga G 109
mega M 106
kilo k 103
deci d 10-1
centi c 10-2
milli m 10-3
micro μ 10-6
nano n 10-9
pico p 10-12
femto f 10-15
2.2 Making measurements and analysing data
2.2.1 Measurements and uncertainties
• True value – value that would be obtained in an ideal measurement
• Measurement error – difference between a measured value and the true value
2|T. Chaudhary
AS PHYSICS – MODULE 2: FOUNDATIONS OF PHYSICS
2.1 Physical quantities and units
2.1.1 Physical quantities
• Physical quantity – property of an object/phenomenon that can be measured
• Some quantities are just numbers e.g. efficiency so just have a numerical value
but most physical quantities have a numerical value and a unit
• Estimates of common physical quantities:
Length of ruler = 30 cm Walking speed = 1 ms-1
Height of a person = 1.5 m Speed of car on a motorway = 30 ms-1
Mass of an apple = 0.1 kg Acceleration of free fall = 10 ms-2
Mass of a person = 70 kg Density of air = 1 kgm-3
Mass of a car = 1000 kg Density of water = 1000 kgm-3
2.1.2 S.I units
• Some quantities have many different units e.g. length has metres, inches and
miles
• Therefore, scientists use a standard system of units called the Systѐme
International d’Unitѐs (International System of Units)
SI base units:
Quantity Base unit Unit symbol
length metre m
mass kilogram kg
time second s
electric current ampѐre A
temperature kelvin K
amount of substance mole mol
• Unit symbol is written in lower case unless unit is named after a person
Derived units:
• Derived units without special unit names:
Derived quantity Derived unit
area m2
volume m3
speed ms-1
acceleration ms-2
density kgm-3
momentum kgms-1
1|T. Chaudhary
, 2
• Derived units with special unit names:
Derived quantity Unit name Unit symbol In other SI units
force newton N kgms-2
pressure pascal Pa Nm-2
energy or work done joule J Nm
power watt W Js-1
electrical potential difference volt V JC-1
electric resistance ohm Ω VA-1
electric charge coulomb C As
frequency hertz Hz s-1
Checking the homogeneity of physical equations using S.I base units:
• If an equation is correct, it must have same units for each quantity (i.e. they must
be homogeneous)
• Base units can be used to check whether an equation representing physical
quantities could be correct
• EG: v = u + at
• Left side: ms-1
• Right side: ms-1 + (ms-2 x s) → ms-2 x s = ms-1 ∴ equation is homogeneous
Prefixes:
• SI uses prefixes to show multiples and fractions of units
• Prefixes for multiples all have initial capitals (apart from kilo)
• Prefixes for fractions are all lower case
Prefix name Prefix symbol Factor
peta P 1015
tera T 1012
giga G 109
mega M 106
kilo k 103
deci d 10-1
centi c 10-2
milli m 10-3
micro μ 10-6
nano n 10-9
pico p 10-12
femto f 10-15
2.2 Making measurements and analysing data
2.2.1 Measurements and uncertainties
• True value – value that would be obtained in an ideal measurement
• Measurement error – difference between a measured value and the true value
2|T. Chaudhary
