Physical Quantities, Units &
Measurement (QUM)
MC textbook, Chp 1 & 4
GLM textbook, Chp 1 & 4
Korean Air 6316
• video:
http://www.youtube.com/watch?v=bfA9Y8CI
GLc
• Pilot heard air traffic controller say 1500 feet
when they said 1500 m
• Instead of climbing they dived and crashed,
killing all 3 crew members
Topics
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What is a Physical Quantity?
Prefixes & Standard Form
Orders of Magnitude
Base Quantities & Units
4 Important Quantities
Vernier Calipers
Micrometer Screwgauge
What is a Physical Quantity?
• Most physical quantities have a
numerical magnitude and a unit
Numerical Magnitude
10 cm
Unit
Prefixes & Standard Form
• Some quantities can be very large or very
small. It then becomes cumbersome to write
these numbers.
• E.g. the radius of the Earth is 6371000 m, the
size of an atom is around 0.0000000001 m.
• There are two ways to simply the writing of
very large or very small quantities: using
prefixes or standard form.
Standard Form
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Radius of Earth = 6371000 m
(in standard form) = 6.37 x 106 m
Size of atom = 0.0000000001 m
(in standard form) = 1.00 x 10-10 m
IMPORTANT!
• For ALL your Physics homework, tests and
exams, the following rule applies:
• Leave all final answers to 3 significant figures
• If the answer is more than 5 digits, convert to
standard form
• If you fail to do so in an exam, marks can be
deducted
Prefixes
Prefix
Symbol
Magnitude
Mega
Kilo
Deci
Centi
Milli
Micro
Nano
M
k
d
c
m
µ
n
106
103
10-1
10-2
10-3
10-6
10-9
Practice Task 1
Orders of Magnitude
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Diameter of Earth (approx): 107 m (or 10 000 km)
Singapore to Thailand: 106 m (or 1000 km)
Singapore to Johor: 105 m (or 100 km)
Height of Mt Everest: 104 m (or 10 km)
Yishun MRT to YCK MRT: 104 m (or 10 km)
Tallest Building in the World: 103 m (or 1 km)
Length of full size football field: 100 m
Thickness of paper: 10-4 m (or 0.1 mm)
Width of human hair: 10-4 m (or 0.1 mm)
Size of red blood cell: 10-5 m (or 10 µm)
Size of Atom: 10-10 m (or 0.1 nm)
Base Quantities & Units
Quantity
Length
Mass
Time
Electric Current
Temperature
Amount of Substance
Unit
metre
kilogram
second
ampere
kelvin
mole
Symbol
m
kg
s
A
K
mol
Significance of Base Quantities
• Why did scientists single out these base
quantities?
• ALL other quantities in science can be derived
from these base quantities!
• E.g. Speed = Distance / Time
• Speed is not a base quantity, but distance and
time are.
Deriving of Units
• If quantities can be derived from base quantities,
units can also be derived from base units.
• E.g. Speed = Distance / Time
• [Units of Speed] = [Units of Distance] / [Units of Time]
• = m/s or ms-1
• IMPORTANT NOTE: in Physics, always use the
negative indices notation instead of the slash
notation
Practice Task 2
QUIZ 1A
Assignment 1A
• [placeholder for due date]
• [reminder of late submission policy]
4 important quantities
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1) Mass
2) Volume
3) Density
4) Weight
Mass
• Definition of Mass: the amount of matter in a
body
• Mass is measured using a beam balance
Mass
• Mass is also a measure of the inertia of an
object
• Definition of inertia: the reluctance of an
object to change its state of rest or motion,
due to its mass.
Volume
• Volume is the amount of space an object
occupies
• How is mass different from volume?
• When we say an object is big, are we referring
to mass or volume?
• How do we measure the volume of an
irregular shaped object?
Density
• Density is a property of a substance
• It doesn’t matter how big or how small an
object is, if it is made of the same substance,
it will have the same density
• The symbol of density is ρ (“roe”)
• The equation for density is:
• ρ = mass / volume
Weight
• Definition: the gravitational force, or gravity,
acting on an object.
• Weight is a force (you will learn more about
forces in Unit 3)
• Weight is the force of the Earth pulling you
downwards (towards the center of the Earth)
• The greater the mass, the greater the weight
Mass vs Weight
• While they are related, mass and weight are
two different quantities
• If an astronaut travels to the moon, his mass
remains the same, but his weight decreases by
6 times!
• Mass is constant, but weight changes
depending on which planet the object is on.
• If an object is in outer space, weight = zero.
Mass vs Weight
• There is another difference between mass and
weight
• Weight is a quantity with a direction (i.e.
downwards). Weight will never point
upwards, or any direction other than
downwards.
• Mass is a quantity with no direction. It is just a
number.
• You will learn more about this difference in
Unit 2.
Measurement of Mass & Weight
• In the lab, when asked to measure the mass of
substances, you will use an electronic balance.
Are we truly measuring the mass?
Practice Task
• Task 1: GLM pg 75, Qn 1
• Task 2: GLM pg 75, Qn 4
TIME FOR QUIZ!
Assignment 1B
• TYS Topic 4
• Paper 1 Qn 1, 2, 4, 5, 10
• Paper 2 Qn 2
• [placeholder for due date]
• [reminder of late submission policy]
Measurement
• We measure quantities using measuring
instruments
• Some common measuring instruments which
you will be using are:
• Length: metre rule, vernier caliper,
micrometer screwgauge
• Mass: Electronic Balance
• Time: Stop watch
• Volume: Measuring Cylinder
• and others (for temperature, electricity, etc.)
Reading vs Measurement
• For some measuring instruments, the reading
you get from the instrument is the
measurement of the quantity (e.g.
thermometer, measuring cylinder)
• For some measuring instruments, there is a
difference between the reading on an
instrument, and the actual measurement
Reading vs Measurement
• Consider the weighing scale below,
what is the actual weight of the man?
60.0
0.2
Reading vs Measurement
• How about the weight of this man?
70.0
- 0.5
Reading vs Measurement
• In the case of the weighing scale, the
measurement is actually the difference
between two readings.
• The reading (with man) minue the reading
(without man)
• Usually the reading without the man should
be zero, but when it is not zero, the
instrument is said to have zero error
Vernier Calipers
• http://www.phy.ntnu.edu.tw/ntnujava/htmltag.php?code=u
sers.ntnu.fkh.caliper2_pkg.caliper2Applet.class&name=calip
er2&muid=2
Practice Task
• Task 1: GLM Pg 13, Qn 4(c)(i)-(iii)
• Task 2: GLM Pg 13, Qn 5(b)(i)-(ii)
Micrometer Screw Gauge
• http://www.phy.ntnu.edu.tw/ntnujava/htmltag.php?
code=users.ntnu.fkh.micrometer2_pkg.micrometer2
Applet.class&name=micrometer2&muid=2
Practice Task
• Task 1: GLM pg 18, Qn 1(c)(i)-(iv)
• Task 2: GLM pg 19, Qn 2(b)(i)-(iii)
TIME FOR QUIZ!
Assignment 1C
• TYS, Topic 1A, Paper 1 Qn 1, 2, 4, 5, 6, 9
• [placeholder for due date]
• [reminder of late submission policy]
Summary
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What is a Physical Quantity?
Prefixes & Standard Form
Orders of Magnitude
6 Base Quantities & Units
4 Important Quantities (Mass, Volume,
Density, Weight)
• Vernier Calipers
• Micrometer Screw Gauge
EXIT CARD