Name
CHAPTER 3
Class
Date
States of Matter
SECTION
3 Fluids
KEY IDEAS
As you read this section, keep these questions in mind:
•
•
•
•
How do fluids exert pressure?
What causes objects to float?
What happens when pressure in a fluid changes?
What affects the speed of a fluid?
What Are Fluids?
Recall that liquids and gases are examples of fluids.
The particles in a fluid can move past each other. Fluids
exert pressure, or push, evenly in all directions. The properties of fluids allow ships to float, divers to explore the
ocean, and jets to soar across the sky.
What Is Pressure?
READING TOOLBOX
Summarize As you read,
write down each scientific
law or principle described in
this section. Write each one
in your own words. Include a
diagram or equation that describes the principle or law.
Pressure is the amount of force exerted on a given
area of surface. For example, when you add air to a bicycle tire, you push air into the tire. Inside the tire, the air
molecules push against each other and also against the
walls of the tire. As you pump more air into the tire, the
pressure inside the tire increases. Why? More air particles
are pushing against the inside of the tire and against each
other.
EHHDBG@<EHL>K
1. Predict What would
happen to pressure in the tire
if some of the air were
removed? Explain your answer.
The force of air particles inside
the tire creates pressure. Pressure
keeps the tire firm.
CALCULATING PRESSURE
You can calculate pressure by dividing the force by the
area over which the force acts. The equation for calculating pressure is on the next page.
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States of Matter
Name
SECTION 3
Class
Date
Fluids continued
Pressure
force
pressure = _____
area
READING CHECK
2. Identify What is the SI
unit of pressure?
P = _F_
A
The SI unit of pressure is the pascal. One pascal
(1 Pa) is the force of one newton exerted over an area of
one square meter (1 N/m2). The newton is the SI unit of
force.
What Causes an Object to Float?
What happens if you push a rubber duck to the bottom
of a tub and release it? It does not sink to the bottom of
the tub. It pops up to the surface. Why does this happen?
A force called the buoyant force pushes the rubber duck
upward. All fluids exert an upward buoyant force on matter. You may feel this force when you float on your back
in a pool or lake.
KXcb8Yflk@k
Discuss According to legend,
Archimedes helped a king
determine if his crown was
made of pure gold or of silver.
With a partner, discuss ways
to use Archimedes’ principle
to help the king learn if his
crown was made of gold.
(Hint: The density of silver is
less than the density of gold.)
DETERMINING BUOYANT FORCE
A Greek mathematician named Archimedes discovered a method to determine buoyant force.
Archimedes’ Principle
The buoyant force on an object equals the
weight of the fluid that the object displaces.
The figure below shows how to find the buoyant force
on an object.
EHHDBG@<EHL>K
3. Identify Relationships
What does the volume of
displaced water equal?
Lower an object into a
container of water.
The object displaces
water. The displaced
water flows into a
smaller container.
When water covers the
object completely, you
can measure the volume of the displaced
water. The volume of
the water equals the
volume of the object.
Once you have collected the displaced water, you can
find its weight. The weight of the displaced water equals
the buoyant force.
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States of Matter
Name
SECTION 3
Class
Date
Fluids continued
DETERMINING IF AN OBJECT WILL SINK OR FLOAT
You can predict if an object will sink or float by comparing its weight with the buoyant force on it. An object
floats when the buoyant force is greater than or equal to
the object’s weight.
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4. Analyze If an object’s
weight is less than the buoyant force, will it sink or float?
You can also compare the densities of a fluid and
an object to predict if the object will float. If the object
is denser than the fluid, it will sink. For example, the
density of a brick is 2 g/cm3. The density of water is
1.00 g/cm3. The brick is denser than water and thus will
sink in water.
Steel is almost eight times denser than water.
However, large ships made of steel can float in the ocean.
Why? The shape of the boat allows it to float. Because a
large part of the boat is hollow, the same mass of steel
takes up a larger volume. Increasing volume decreases
overall density and the boat floats.
EHHDBG@<EHL>K
5. Explain Why does a block
of steel have a larger density
than a boat made of steel?
A large part of the boat is hollow. Although shape
does not change the mass of the boat, the volume of
the hollow boat is larger. A larger volume for a particular mass reduces the density of the boat.
A block of steel is
denser than water,
so the block sinks.
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States of Matter
Name
SECTION 3
Class
Date
Fluids continued
What Happens When Pressure Changes in a
Fluid?
What happens when you squeeze one end of a tube of
toothpaste? The pressure you apply at one end is passed
along throughout the entire tube. This causes toothpaste
to come out the other end. In the 17th century, a French
scientist named Blaise Pascal explained this kind of
observation.
Pascal’s Principle
If the pressure in a container is increased at any point,
the pressure increases at all points by the same amount.
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6. Identify What do the variables P, F, and A represent?
Pascal’s principle can be stated by the following
equations:
Pascal’s Principle
P1 = P2
P = _F_
A
F
F
Therefore, ___1 = ___2
A1 A2
HYDRAULIC DEVICES
Hydraulic devices are based on Pascal’s principle.
Hydraulic devices use liquids to transmit, or pass,
pressure from one point to another. They can be used
to multiply force.
Math Skills
7. Calculate A hydraulic lift
uses Pascal’s principle to lift
a 19,000 N car. The area of
the small piston equals
10.5 cm2 and the area of
the large piston equals
400 cm2. What force must
you exert on the small
piston to lift the car?
A small downward force is applied
to a small area. This force exerts
pressure on the fluid in the device.
Force 1
Area 2
Area 1
The pressure is
transmitted to a
larger area, where
the pressure creates
a larger force.
Force 2
According to Pascal’s principle, the pressure is the same on both sides of the
fluid in the device. Therefore, a small force on a smaller area produces a larger
force on the larger area.
What Are the Properties of Fluids in Motion?
Examples of moving fluids include liquds flowing
through pipes, air moving as wind, and honey dripping.
Fluids can flow at different rates. However, fluids in
motion have some properties in common.
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States of Matter
Name
Class
SECTION 3
Date
Fluids continued
FLUIDS AND AREA
If the flow rate stays the same, fluids move faster
through small areas than through large areas. For example, if you place your thumb over the end of a garden
hose, your thumb blocks part of the opening. The water
must flow through a small area. Because the area is
small, the water leaves the hose at a faster speed.
READING CHECK
8. Describe How does area
affect the speed of a fluid?
FLUID PRESSURE AND SPEED
Imagine water carrying a leaf through a pipe. The
water moves faster through the narrow part of the pipe
than through the wider part. Therefore, the leaf carried
by the water moves more quickly in the narrow part of
the pipe. The water pressure behind the leaf is greater
than the pressure in front of the leaf. The pressure difference causes the leaf to speed up, or accelerate, as it
enters the narrow tube.
More pressure
Less pressure
Pressure is lower in the narrow
part of the pipe than in the
wide part. As the leaf enters
the narrow part, the pressure in
front of the leaf is less than the
pressure behind it. Thus, the
leaf speeds up when it enters
the narrow part of the pipe.
EHHDBG@<EHL>K
9. Identify On the diagram,
write the words slower and
faster to show how speed
differs in different parts of
the tube.
A Swiss mathematician named Daniel Bernoulli
described this property of a moving fluid.
Bernoulli’s Principle
As the speed of a moving fluid increases, the
pressure of the moving fluid decreases.
VISCOSITY
Some fluids move much more slowly than others. For
example, it would take you longer to pour a cup of honey
than a cup of water. Another way to say this is that different fluids have different viscosities. Viscosity is a fluid’s
resistance to flow. A viscous fluid does not flow as easily
as a less viscous fluid. In general, the stronger the attraction between the particles of a fluid, the more viscous the
fluid is.
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States of Matter
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Section 3 Review
SECTION VOCABULARY
buoyant force the upward force that keeps an
object immersed in or floating on a fluid
pascal the SI unit of pressure; equal to the
force of 1 N exerted over an area of 1 m2
(symbol, Pa)
pressure the amount of force exerted per unit
area of a surface
viscosity the resistance of a gas or liquid to flow
1. Describe How can you use density to determine if an object will sink or float in a
fluid?
2. Analyze You put a small object into a cup of water and weigh the displaced water.
The displaced water weighs 235 N. What is the buoyant force on the object?
Explain your answer.
3. Explain How does a hydraulic device multiply force?
4. Infer A balloon filled with helium floats in the air. What does this tell you about
the density of helium?
5. Explain How are speed and pressure of a fluid related?
6. Explain How do the attractions between particles in a fluid determine viscosity?
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States of Matter