Chapter 4 The Properties of Fluids
4.1 A Close up look at Fluid Flow
 Fluids are things that flow. They flow because something is pushing or
pulling on them. On earth this is typically gravity but can also be a pump.
 How quickly a fluid flows is called flow rate.
 Things that are moving are called dynamic. So aerodynamic means
moving air and hydrodynamic means moving water.
 Sometimes fine powders (sugar) seem like fluids but unlike most fluids
they can pile. (talk about wettability and how water can pile)
 The kinetic molecular theory states that all matter is made of tiny
particles in motion. In solids the matter is rigid and doesn't move much, it
keeps shape and volume. In liquids the particles can slide past each other
allowing them to take the shape of the container. In gases they move past
each other and spread out to fill any container they are in.
 As you add energy the particles in a solid move more and more until the
slide past each other (melting). Then as you add more energy they
eventually break away from each other (evaporation). The reverse
happens as you cool them. Gas to liquid (condensing), liquid to solid
(solidification). If you add enough heat at the right air pressure you can go
straight from solid to gas (sublimation)
4.2 fluid flow around objects
 The shape of an object determines how fluids flow around it.
 Fluids flow smoothly around some objects. These are called streamlined
objects and the smooth almost straight flow is called laminar flow.
 Fluid flows really choppily around some blocky objects. This flow is called
turbulent flow.
 Even smooth objects interact with fluid flow a little bit. The amount they
"grab" at the fluid sliding past them is called drag.
 To get a picture of how fluid flows around an object engineers use
something called a wind tunnel. It allows them to "see" the flow of air by
adding smoke or paper strips.
4.3 Viscosity: A Property of Fluids (pg 107)
 Some fluids are hard to start pouring, it seems to require more force to
get some fluids to start. Why is this? Let’s consider a few liquids that we
are familiar with like ketchup, water, vinegar, corn syrup.
Liquid
Force required to make it flow.
Ketchup
Water
Vinegar
Corn Syrup
 When we take a look at which fluids needed more force we start to see
a pattern it seems that the “thicker” the fluid is the more force it needs.
We call this resistance to flowing and movement viscosity.
 Remember the KMT? The attraction between the particles in a fluid are
called the cohesion. A strong cohesion would make a fluid more viscous.
 There is also attraction between particles in a fluid and the container
they are touching. This is called adhesion.
 You may have noticed if a pudding is left out it forms a skin across the
top. All fluids have a thin skin at the surface that holds tightly together.
This is called surface tension.
 So how do we measure viscosity? We can measure how long it takes for
a liquid to flow out of a container through a standard sized hole.
Scientists use a tool like this called a viscometer.
4.5 Measuring Matter: Mass, weight, and Volume (pg 113)
 In order to understand more about fluids we need to know a few things
about how to measure matter.
 The first thing is the difference between mass and weight. We often
treat them the same but they are not.
o Mass is the amount of matter in an object. If the object doesn’t
change then the mass would be the same on the earth or the
moon. It is measured in grams.
o Weight is the amount that gravity pulls down on you. We
compare the strength of gravity of other planets to Earth. For
example on the Moon gravity is 1/6 of Earth’s. That means that
you would divide your weight by 6 to find what you weigh on the
Moon. Weight is measured in Newtons.
 Another thing that we measure about matter is volume. Matter takes up
space. We call this space volume. The easiest way to figure out how
much space something takes up is by displacement.
 Displacement is when you push an object underwater and it causes the
water level to go up, like when you get into a tub. We can measure the
volume of something by measuring the level of water before and after
displacement. The change in the level is the volume of the object.
o Often we measure volume in a graduated cylinder. When we do
this we need to read the meniscus of the water (the part where it
curves down).
4.7 Density: Another Property of Fluids (pg 119)
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When we pour oil on top of water we see that the oil floats on top.
This is useful but why does it happen? We can say it is lighter but if
you only had 10 ml of water and 90 ml of oil the oil is definitely
heavier and yet it still floats on top.
The light and heavy idea seems right but we have to make sure that
we compare the same volume of fluid.
When we compare similar volumes then it makes sense to also
compare their masses. We call the combination of these two things
density. Density is mass divided by volume:
𝑚𝑎𝑠𝑠
𝑚
o 𝐷𝑒𝑛𝑠𝑖𝑡𝑦 =
𝐷=
𝑣𝑜𝑙𝑢𝑚𝑒
𝑉
For density we usually measure mass with grams (g) and volume with
cm3 (ml), giving density a unit of g/cm3 or g/ml.
The handy thing about this kind of formula is that if you know any
two pieces of it you can find the third.
𝑚
𝑚
o 𝐷 =
𝑚=𝐷×𝑉
𝑉=
𝑉
𝐷
To find the density of something we just need to measure its volume
and its mass. Then we can calculate its density.
Water has a density of 1.00 g/ml. If an object has a density that is less
than this it will float on the water. If it has a density greater than water
then it will sink.
4.9 Comparing Densities (pg 125)
 Usually solids have a greater density liquids and liquids a greater
density than gases. This makes sense when you recall the kinetic
molecular theory. As matter heats up it spreads out taking up
more volume. Liquids take up more volume than solids and
gases more volume than liquids.
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Since density for a substance changes with temperature we
need to give the temperature when we give the density. If no
temperature is given then it is for room temperature.
There are exceptions to the solids are most dense rule, mercury
and water are two of these.
 Mercury is a liquid metal that is 13 times more dense than
many solids.
 Liquid water is more dense than solid water (ice).
You can compare the density of ice to liquid water by looking at
an ice cube floating in water. The ice cube is 90% submerged.
This means that ice most have only 90% of the density of liquid
water.
1.10 The ups and Downs of Buoyancy (pg 127)
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Remember displacement? When something goes into the water it
pushes the water out of the way. Well the water pushes back on the
object from all directions. The sideways pushes cancel out but the
upward force is called the force of buoyancy.
(Draw this on the board)
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Buoyancy is not the only force acting on an object. Gravity also pulls
the object down. If the force of buoyancy is greater than the force of
gravity then the object will float. If the force of buoyancy is less than
the force of gravity then the object will sink.
We can measure the force due to gravity but how can we know the
size of the force of the buoyancy? Using something called
Archimedes' Principle:
o The buoyant force on an object
immersed in a fluid is equal to the
weight of the fluid that the object
displaces.
4.11 how and Why do things float?
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In order for something to float it must have a density that is less than
the fluid it is floating on. So how do you make a metal boat float?
You change the density by making the object take up more volume. It
still has the same mass but the greater volume means it has a lower
density on average. A boat does this with its shape.
What forces are acting on floating objects? Well gravity is pulling
them down and the water being pushed out of the way by the hull is
pushing back up on it (buoyancy).
If that upward force, buoyancy, is greater than the object will float.
Different boats float higher in the water than others. How come?
Well Archimedes Principle states that the buoyancy force is equal to
the weight of the water displaced. This means that the more water a
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boat displaces the stronger the water will push back on it. (try to
push and hold a balloon under water, inflated and deflated, which is
harder?)
If an object is being pushed up by the water more than being pulled
down by gravity we call that positive buoyancy.
If an object is being pushed up by the water, and pulled down by
gravity equally then it is neutral buoyancy.
If an object is being pushed up by the water less than it is being
pulled down by gravity then it is negative buoyancy.
You can have buoyancy in the air as well. If an object is less dense
than the air it will float.
As you add weight to a boat it floats lower in the water. Boats have
Plimsoll lines that mark off the point where you should stop loading
the ship to keep it from sinking.
4.12 How does Temperature affect Viscosity and Density?
 Obviously honey does not pour well when it comes out of the fridge.
Fluids flow quicker when they are warm. It seems that viscosity, density
and buoyancy all change with temperature.
 As heat is taken away from a liquid its particles slow down. As they slow
down their more likely to stick closer and tighter together. This means
that their volume shrinks. What would that do to their density?
𝑚
o 𝐷=
if m = 1 and v = 1 to start then d = 1. Let’s keep the
𝑣
others the same and shrink v. 𝐷 =
1
0.5
=2
 As you can see as the volume shrinks the density increases. This means
that for liquids other than water as they cool their density will increase.
 What does a higher density mean for buoyancy? Buoyancy depends on
how much water is displaced. If the volume of the object decreases but
the mass stays the same it is going to displace less water. As it displaces
less water it will have less buoyancy.
 Finally what does it mean for the viscosity? Well if the atoms are sticking
closer together it means that they are more likely to stick together. This
will cause the viscosity to go up.
 So why is water weird? It does the opposite, when it gets colder it floats.
In fact it is densest at 4oC.
 It turns out that water is shaped like a V. This means that when it is a
liquid the v’s can pack in tightly around each other. But when it is in
solid form the V shape spreads it out to take up more volume.
Chapter 5 The Use of Fluids
5.4 Fluids Under Pressure
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Pressure can be calculated by using the formula Pressure =
𝐹𝑜𝑟𝑐𝑒
𝐴𝑟𝑒𝑎
or P =
𝐹
𝐴
Force is measured in Newtons (N)
o Area is measured in units2
o Pressure is measured in Pascals (P) or Kilopascals (kPa)
 Atmospheric pressure decreases the higher you go and increases as you
descend below sea level.
o
5.5 Pressure in Confined Fluids
 There are two types of systems that use confined fluids
o Hydraulic
 A confined pressurized system that uses moving liquids
o Pneumatic
 A confined , pressurized system that uses moving air or
other gasses such as carbon dioxide
 gasses can be compressed which allows the particles of the gas to be
pressurized pushing the particles closer together reducing the volume
5.8 Fluid Power at Work for Us
 Both Hydraulic and pneumatic systems can be helpful in many ways.
 Both systems have unique properties which allow the impossible to
happen, such as a person lifting a very heavy object with little effort.
Fluids and Living Things
6.1 From Bladders to Ballast
 There are both natural and engineered methods of altering buoyancy
 Fish use their swim bladder
 people use ballasts, wet suits and buoyancy compensators
6.3 Pressurized Fluid System: Hydraulics
 Hydraulics use liquids (hydraulic oil) under pressure to move things
 uses a cylinder and a piston to move objects
o Oil fills up the cylinder pushing the piston out of the cylinder
 A pump is used to pressurize the system.
 Our heart is a natural hydraulic system
o knowledge of how our heart works has led to designs of artificial
hearts
6.04 Pressurized Fluid System: Pneumatics
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Pneumatics use pressurized gasses used to move things.
An air compressor is used to build up pressure
Used a lot in Construction industries.
Our lungs are an natural pneumatic system