C H P 2

PROPERTIES, QUANTITIES & MEASUREMENT

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=en-GB&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)

IMPORTANT PHYSICAL QUANTITIES

• 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. old-grandfather clocks)• What is the principle behind quartz clocks and

watches?

SUMMARY

Quantity S.I. Unit

Measured by

Length m ruler, vernier calipersArea m2 (estimated)

Volume m3 measuring cylinder, Eureka can

Mass kg beam balance, mass balance

Density kgm-3 (calculated)Temperature K thermometer

Time s 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

time/s

5

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

time/s

20

5

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

SUMMARY

• Physical Properties• Strength• Hardness• Flexibility• Electrical Conductivity• Thermal Conductivity• Boiling Point• Melting Point• Density

• Physical Quantities• Units & Prefixes

SUMMARY

• Physical Quantities (cont’d)• Length• Area• Volume• Mass• Density• Temperature• Time

• Distance-Time Graphs• Uniform Speed• Non-Uniform Speed• Ticker-Tape • Average Speed