Grade 8
Discovering Science
Unit 3: Fluids and Viscosity
Chapter 7
Viscosity describes a fluid’s resistance
to flow.
QUESTIONS
• Why do some fluids seem to hold their shape
longer?
• Why do some fluids take up more space than
other fluids at different t temperatures?
• Why do some fluids flow slowly while other
flow quickly?
The Particles Theory
• All matter is made up of particles.
• These particles are constantly moving – they
have energy.
• There are spaces between these particles.
• There are attractive forces between the
particles.
• The particles of one substance differs from the
particles of other substances.
The States of Matter
Properties
State
Shape
Solid
Fixed
Liquid
Gas
Volume
Does not
change
Unfixed Does not
change
Unfixed Changes
Particle
Arrangement
Particle
Movement
Tight
Vibrate
Tight
Flowing
Loose
Random
The Changing States of Matter
•
•
•
•
•
•
•
•
Melting
Freezing
Evaporation
Condensation Sublimation
Deposition
Ionization
Deionization -
Solid to liquid
Liquid to solid
Liquid to gas
Gas to liquid
Solid to gas
Gas to solid
Gas to plasma
Plasma to gas
Changing States of Matter - Continued
Fluids
• A fluid is anything that flows.
• Example:
Water, maple syrup, milk, motor oil, Cheese
Whiz, compressed air in tires, ...
Viscosity
• The viscosity of a fluid is related to the
amount of friction between particles
Friction is the resistance to movement.
• Viscosity describes how “thick” or “thin”
something is.
• Viscosity is how easily a fluid flows.
Flow rate is the amount of fluid that flows
past a point in a given amount of time
Examples Viscosity
• Motor oil, in the winter, flowing from its
container flows slow, but in the summer it
flows fast.
• Pancake syrup, just out of the refrigerator,
flowing from the bottle flows slow, but when
warmed up by placing it under warm water
will flow fast.
** relate to the particle theory.
The particle Theory and Viscosity
Several factors influence the viscosity of a fluid,
they include:
• Strength of attractive forces
• Temperature variations
• Concentration of the fluid
• Particle size
Chapter 8
Density describes the amount of
mass in a given volume of a
substance
Density
• Density is the amount of mass in a certain unit
volume of a substance.
• A bowling ball has a high density of particles
in comparison to a balloon filled with helium
gas or a glass of water.
• * The particle theory, density and a thermometer.
Density cont...
** Solid objects can move easily through liquids
and gasses, while gases and liquids find it difficult
to travel through solids, if at all.
• Gasses are less dense than liquids
• Liquids are less dense than solids
* Displacement is the amount of space that an
object takes up when placed in a fluid.
Calculating Density, Volume and Mass
• Density is the amount of mass in a certain unit
volume of a substance.
• Volume is a measurement of the amount of
space occupied by a substance.
• Mass is the amount of matter in a substance.
Calculating Density
Formula:
Word Problem:
• Density (D) = Mass (m) /
Volume (V)
• Find the density of a
substance with a mass of 25
g and a volume of 4 cm3
D = m/V
• D = m/V
• D = 25 g / 4 cm3
• D = 6.25 g/cm3
Calculating Volume
Formula:
Word Problem:
• Volume (V) = Mass (m) / Density
(D)
• Find the volume of a
substance with a density of
8.5 g/ cm3 and a mass of 34
g.
V = m/D
• V = m/D
• V = 34 g / 8.5 cm3
• V = 4 cm3
Calculating Mass
Formula:
Word Problem:
• Mass (m) = Volume (V) x
Density (D)
• Find the mass of a
substance with a density of
10.2 g/ cm3 and a volume
of 3.4 cm3.
m = VD
• m = VD
• m = 3.4 cm3 x 10.2 g/ cm3
• m = 34.68 g
Density Changes in Everyday Life
Examples: (Discuss)
• A baking cake
• Water (three states)
• Warms and cool tire pressure
• Drying wood
• Hot air balloons
• Salt water
Chapter 9
Forces influence the motion and
properties of fluids
Force
• A force is a “push” or “pull”.
• A force can start an object to move or stop an
object from moving.
• A force can cause an object to speed up or
slow down.
• A force can change the direction of a moving
object.
• A force can be a bend, twist of squeeze.
• An arrow is used to symbolize force
Types of Forces
• Buoyancy
• Magnetic
• Gravity
• Friction
Forces
Balanced Forces:
Unbalanced Forces:
• Balanced forces are equal to
each other and opposite in
direction to each other.
• Potential energy - stored
energy; energy at rest.
• One force is greater than
another
• Kinetic energy – energy in
motion/
Mass vs. Weight
Mass:
Weight:
• Mass remains the same no
matter where an object is in
the universe.
• Weight is the measure of
the push or pull, of gravity
on an object.
• Weight is measured in units
of Newton (N).
• The weight of an object on
the moon is 1/6 of that on
Earth. A 600 N person
would weigh 100 N on the
moon.
Buoyancy – The “Anti-Gravity” Force
• Buoyancy is the upward force of an object that is
submerged or floating in/on a fluid.
• In other words buoyancy is the upward push of a
substance away from the natural pull of center of the
Earth.
Objects Which Have Buoyant Forces
Acting Upon Them
•
•
•
•
•
Boat
Hot air Balloon
Airplane
Swimmer
Parachutist
Archimedes’ Principle
• The buoyant force acting on an object equals the weight
(force of gravity) of the fluid being displaced by the object.
• If the force of gravity pulling down on an object is equal to
the amount of buoyant force pushing up then the object is
said to have neutral buoyancy; it will neither rise or sink.
Salt Water vs. Fresh Water
• Fluids with high density (particles are closer
together) exert a greater buoyant force than fluids
with low density (particles farther apart from each
other).
• Example: Salt water is more dense than fresh water,
therefore, object is salt water will float better than is
fresh water.
Design Influences an Object’s Ability
to Sink or Float
• Concrete or steel boat vs. a chunk of concrete or a
section of steal.
• A silk/canvass hot air balloon vs. a sheet of canvass.
• Submarine vs. an iron pole.
Average Density
• The average density of an object is the total mass of
all substances that make up the object divided by the
total volume of the object.
• Ships can be built of steel because the haul of the
ships can be constructed large enough to ensure the
density of air in the haul is low.
Pressure
• Pressure is the force acting on a certain area
of a surface.
• Example: When you place your finger on a
bruise, you are applying pressure to that
specific area. You are not applying pressure to
your ankle, ... Just the bruise.
Force, Area and Pressure
Two General Conclusions
1.) The larger the force, the greater the
pressure.
2.) The smaller the area, the greater the
pressure.
Calculating Pressure
Formula:
Word Problem:
•
• A serving jug hold 500 N of
Kool-Aid. If the base of the
serving jug is 0.05 m2, what
pressure does the Kool-Aide
exert on the base of the
serving jug?
P=F/A
P = 500 N / 0.05 m2
P = 10 000 Pa
Pressure is calculated by measuring
the force that is being exerted on
an object and dividing it by the area
over which the force is being
exerted.
• Pressure (P) = Force (F) / Area
(A)
P=F/A
• Pressure is measured in units
of pascals (Pa)
Calculating Force
Formula:
• Force (F) = Pressure (P) x
Area (A)
F=PxA
Word Problem:
• If 63 000 Pa of pressure is
being exerted on a rubber
ducky with a area of 0.09
m2, how much force is being
exerted on the rubber
ducky?
• F=PxA
• F = 63 000 Pa x 0.09 m2
• F = 5670 N
Calculating Area
Formula:
• Area (A) = Force (F) /
Pressure (P)
A=F/P
Word Problem:
• If 500 N of force is being
exerted on a balloon
creating pressure of 1600
Pa, what is the area of the
object?
• A=F/P
• A = 500 N / 1600 Pa
• A = 0.3125 m2
Pressure, Volume and Temperature
• Increasing the temperature of a gas increases the
volume of the gas (pressure being held constant).
• Increasing the temperature of a gas increases the
pressure of the gas (volume being held constant).
• Increasing the pressure of a gas decreases the
volume of a gas (temperature being held constant).
Compression vs. Incompressibility
• Compression
The ability to squeeze into a smaller volume.
ex. Aerosol cans
• Incompressibility
The inability to squeeze into a smaller volume .
ex. Try squeezing a unopened 2l Pepsi bottle. What
happened… did not happened?
Pascal’s Law
• Pascal’s law states that pressure applied to an
enclosed fluid is transmitted with equal force
throughout the entire container.
Discuss:
• Compressed air
• Propane cylinders
• Aerosol cans
Application of Pascal’s Law
• A car lift
• An hydraulic jack
• Automatic breaking system
Hydraulics vs. Pneumatics
• Hydraulics is the study of pressure on “fluids”.
• Pneumatics is the study of pressure on
“gases”.
New Technologies
Hydraulic System
Pneumatic System
• Devices which
exerts/transmits force on a
continuous, enclosed liquid.
• Examples: dentist chair,
dump truck, loaders, car
lifts, human circulatory
system...
• Devises which
exert/transmits force on
continuous enclosed gas.
• Examples: Air compressors,
jack hammers, air nail gun,
...