Grade 8Discovering 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

PropertiesState Shape Volume Particle

ArrangementParticle Movement

Solid Fixed Does not change

Tight Vibrate

Liquid Unfixed Does not change

Tight Flowing

Gas Unfixed Changes Loose Random

The Changing States of Matter

• Melting - Solid to liquid• Freezing - Liquid to solid• Evaporation - Liquid to gas• Condensation - Gas to liquid• Sublimation - Solid to gas• Deposition - Gas to solid• Ionization - Gas to plasma• Deionization - 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 particlesFriction 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:

• Density (D) = Mass (m) / Volume (V)

D = m/V

Word Problem:• Find the density of a

substance with a mass of 25 g and a volume of 4 cm3

• D = m/V• D = 25 g / 4 cm3

• D = 6.25 g/cm3

Calculating Volume

Formula:• Volume (V) = Mass (m) / Density

(D)

V = m/D

Word Problem:• Find the volume of a

substance with a density of 8.5 g/ cm3 and a mass of 34 g.

• V = m/D• V = 34 g / 8.5 cm3

• V = 4 cm3

Calculating Mass

Formula:• Mass (m) = Volume (V) x

Density (D)

m = VD

Word Problem:• Find the mass of a

substance with a density of 10.2 g/ cm3 and a volume of 3.4 cm3.

• 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:• Balanced forces are equal

to each other and opposite in direction to each other.

• Potential energy - stored energy; energy at rest.

Unbalanced Forces:• One force is greater than

another• Kinetic energy – energy in

motion/

Mass vs. Weight

Mass:• Mass remains the same no

matter where an object is in the universe.

Weight:• 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 PressureTwo General Conclusions

1.) The larger the force, the greater the pressure.

2.) The smaller the area, the greater the pressure.

Calculating Pressure

Formula:• 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)

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 / AP = 500 N / 0.05 m2

P = 10 000 Pa

Calculating Force

Formula:• Force (F) = Pressure (P) x

Area (A)

F = P x A

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 = P x A• 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

• CompressionThe ability to squeeze into a smaller volume.ex. Aerosol cans

• IncompressibilityThe 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• Devices which

exerts/transmits force on a continuous, enclosed liquid.

• Examples: dentist chair, dump truck, loaders, car lifts, human circulatory system...

Pneumatic System• Devises which

exert/transmits force on continuous enclosed gas.

• Examples: Air compressors, jack hammers, air nail gun, ...