PROPERTIES,
QUANTITIES &
MEASUREMENT
CHP 2
TOPICS
• 1) Some common properties of matter
• 2) Some important quantities & their measurement
• 3) Speed & Distance-time graphs
PHYSICAL PROPERTIES
WHAT IS A PROPERTY?
• A physical property of a substance (or material) is
something which describes the substance, AND is
true no matter which sample of the substance
• E.g. water freezes at 0 °C
STRENGTH
• How much a substance can support a heavy load
without breaking
• (note: avoid using the word “weak”)
• E.g. Concrete, Steel
HARDNESS
• Harder substances can scratch substances which
are less hard
• (note: avoid using the word “soft”)
• The hardest substance on Earth is diamond
FLEXIBILITY
• The ability to bend without breaking and to return
to its original shape and size
• Related properties:
• A substance is ductile if we can stretch it without
breaking
• A substance is malleable if we can press it without it
breaking (i.e. it flattens)
• Uses?
ELECTRICAL CONDUCTIVITY
• We use the word “conductors”, “insulators” or
“conductivity” in two ways. To avoid confusion, use
the term “electrical conductivity” or “thermal
conductivity”
• Electrical conductivity is how easily electricity
passes through it
• A substance which is a very poor conductor is
called a good insulator
• Metals, water and graphite are good conductors of
electricity
• Most are materials (wood, rubber, etc) are insulators
THERMAL CONDUCTIVITY
• Thermal conductivity is a measure of how easily
heat passes through it
• Objects which are not good conductors are called
insulators
• Metals are good conductors of heat
• Most other materials (e.g. plastic, water) are good
insulators of heat
• The best insulator of heat is actually vacuum
• In a frying pan, do you want to use a good
conductor or good insulator of heat?
MELTING POINT
• “Freezing Point” means the same thing
• The temperature which a pure substances melts
(from solid to liquid)
• Impure substances do not melt at a fix temperature,
but instead over a range of temperatures
• Melting point of water is 0° C
• What is the lowest melting point of any substance?
• Liquid helium freezes at -273 °C
• Note: some substances do not melt into liquid, but
turns directly to gas.
BOILING POINT
• The temperature at which a pure substance boils,
and change its state from liquid to gas
• Note: boiling is different from evaporation
• Substances which are liquid at room temperature
but have a low boiling point (e.g. alcohol) are also
volatile (evaporate easily)
DENSITY
• Video: density tower
(http://www.youtube.com/watch?gl=SG&hl=enGB&v=-CDkJuo_LYs)
• How do you explain the Cartesian Diver?
• Given your knowledge of the Cartesian Diver, can
you explain how a submarine works? (i.e. it is able
to sink or float whenever it wants to)
DENSITY
• Density is the property which determines whether
an object floats or sinks in a liquid
• Objects with greater density than the liquid sink in
that liquid while objects with lesser density float
• If a liquid has very high density, objects float more
easily. This explains why it is easy to float in the dead
sea.
• Video: will a bowling ball float in the dead sea?
http://www.youtube.com/watch?v=ZhL68D9BPiw
• Density is not only a property, it is also a quantity
(next part of the lesson)
PHYSICAL QUANTITIES
WHAT IS A PHYSICAL QUANTITY?
• The term quantity means “number”. (e.g. quality vs
quantity)
• A physical quantity is a number representing an
object
• Sometimes physical quantities can be measured
directly
• Other times, physical quantities may only be
derived through calculations
UNITS
• Most physical quantities have a unit. E.g. a unit for
temperature is °C, a unit for distance is metres
• However, some quantities have more than one unit
(e.g. units of length include metre, kilometre, inches,
feet, cubits, mile, etc.)
• So one day, a group of scientists came together
and said that there should be an official unit for
everything (and other units are unofficial unit)
• The official units are called S.I. Units
• Original plan if for the whole world to use S.I. Units,
but even today, this has not been accomplished
yet
PREFIXES
What’s the difference between these units:
kilometre, metre, centimetre, millimetre
Those that are underlined are called prefixes
They adjust the main unit (metre) when the quantity
is very large or very small
• Refer to your notes for a table of different prefixes
(from Giga to Nano)
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IMPORTANT PHYSICAL QUANTITIES
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Length (distance)
Area
Volume
Mass
Density
Temperature
Time
LENGTH
• S.I. Unit for length is the metre (m)
• You need to be familiar with the ruler, measuring
tape and the vernier calipers to measure length
• To what precision can the ruler measure length?
• To what precision can the vernier calipers measure
length?
• What is one way where we can make an error in
measuring length?
• Parallax Error (refer to notes)
AREA
• S.I. Unit for area is square metre (m2)
• It is difficult to measure area directly
• We usually calculate it (if it is a regular shaped
object) or we estimate it (irregularly shaped object)
VOLUME
• S.I. Unit for volume is cubic metre (m3)
• Other commonly use units: litres, millilitres, pints, fluid
ounces
• We measure volume of liquids using a measuring
cylinder
• We can calculate volume for regularly shaped
objects
• How do we measure volume for irregularly shaped
objects?
• We can use a displacement can(Archimedes can,
Eureka can)
MEASURING CYLINDER
• How to read measuring cylinder:
• Read the lowest point of the meniscus – be
careful of parallax error!
• Read to half the smallest division (if smallest
division is 1 ml, then half smallest division is
0.5 ml)
• We will try this in a later practical
MASS
• Mass in the measure of the amount of substance in
an object
• S.I. Unit is kg
• We measure mass using a beam balance:
MASS
• What’s the difference between mass and weight?
• Weight is the amount of gravity pulling you
downwards towards Earth
• When an astronaut goes to the moon, his mass
remains the same, but his weight decreases by 6
times!
• In other space (zero gravity), your weight is zero!
MASS
• In the lab you will be using an electronic balance to
measure mass
• In reality, what you’re doing is measuring the weight
• But we approximate the weight to be the mass,
since it is more convenient than using a beam
balance
UNITS FOR DENSITY
• The S.I. units for density is kgm-3
• But usually we don’t measure things which are
smaller, so another unit we commonly use for
density is gcm-3
• 1 gcm-3 is 1 g divided by 1 cm3
• 1 kgm-3 = 1000 gcm-3
DENSITY
• Density is the mass per unit volume of an object
• It describes how much matter is squeezed into an
amount of space
• Formula: density = (mass)/(volume)
• S.I. Unit for density is kg/m3 or kgm-3
• It is difficult to measure density directly, usually we
measure the mass and the volume, and divide the
two
• Recall that density is also a property (the same
substance will also have the same density)
LIQUID-IN-GLASS THERMOMETER
• Consider the red liquid (alcohol) in the thermometer
below
• When the temperature increases, what is
happening to the alcohol?
• What is happening to the volume of alcohol?
• What is happening to the density of alcohol?
TEMPERATURE
• The S.I. Units for Temperature is Kelvin (K)
• However, °C is more commonly used (in real life as
well as the lab as well)
• In the lab, we measure temperature using an
alcohol thermometer
• Note: in the past we use mercury thermometers. Be
careful if you ever handle a mercury thermometer;
do not break it, mercury is toxic
• How does an in-ear thermometer work?
TIME
• The S.I. Units for time is seconds (s)
• In the lab we use a stopwatch to measure time.
Record time from a stopwatch to 1 decimal place
• In the lab you have already done the pendulum
experiment
• Did you notice: the time taken for one complete
oscillation (period) is approximately constant, if the
length of the pendulum is constant? This is the
principle behind pendulum clocks (i.e. oldgrandfather clocks)
• What is the principle behind quartz clocks and
watches?
SUMMARY
Quantity
Length
Area
Volume
S.I. Unit
m
m2
m3
Mass
kg
Density
Temperature
Time
kgm-3
K
s
Measured by
ruler, vernier calipers
(estimated)
measuring cylinder,
Eureka can
beam balance, mass
balance
(calculated)
thermometer
stopwatch
DISTANCE-TIME GRAPHS
DISTANCE-TIME GRAPHS
• In the previous unit, you have been introduced to
graphs and graph-plotting
• now we will pay particular attention to a specific
graph: distance-time graph
• This graph represents the distance travelled by an
object with respect to time
• Vertical axis: distance
• Horizontal axis: time
• Note: usually, a distance-time graph is NOT the
graph of a scientific experiment.
STATIONARY
• Stationary objects appear as a flat line
distance /m
5
time/s
UNIFORM SPEED
• Uniform speed means the speed of the object is
constant (not changing)
• Objects travelling with uniform speed appear as a
straight line going upwards
distance /m
20
5
time/s
NON-UNIFORM SPEED
• Non-uniform speed means the speed of the object
is not constant
• It may either be going faster and faster, or slower
and slower
distance /m
distance /m
time/s
time/s
TICKER TAPE
• The ticker-tape is a machine which draws dots at a
regular frequency
• The faster the machine is travelling, the further apart
the dots are
• When the ticker-tape machine is moving at a
uniform speed, it will draw dots which are equally
spaced apart
• If the ticker-tape machine is going faster and faster,
it will draw dots further and further apart
• If the ticker-tape machine is going slower and
slower, it will draw dots closer and closer together
TICKER TAPE
AVERAGE SPEED
There are two ways to understand speed:
1) instantaneous speed
2) average speed
When we say an object is getting faster and faster,
we are referring to its instantaneous speed
• However, even if an object does not have uniform
speed, we can still calculate its average speed
given this formula:
• Average Speed = Total Distance / Total Time
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SUMMARY
• Physical Properties
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Strength
Hardness
Flexibility
Electrical Conductivity
Thermal Conductivity
Boiling Point
Melting Point
Density
• Physical Quantities
• Units & Prefixes
SUMMARY
• Physical Quantities (cont’d)
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Length
Area
Volume
Mass
Density
Temperature
Time
• Distance-Time Graphs
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Uniform Speed
Non-Uniform Speed
Ticker-Tape
Average Speed