States of Matter
Section 3
Section 3: Fluids
Preview
• Key Ideas
• Bellringer
• Pressure
• Buoyant Force
• Comparing Weight and Buoyant Force
• Pascal’s Principle
• Math Skills
• Fluids in Motion
States of Matter
Section 3
Key Ideas
〉How do fluids exert pressure?
〉What force makes a rubber duck float in a
bathtub?
〉What happens when pressure in a fluid
changes?
〉What affects the speed of a fluid in motion?
States of Matter
Section 3
Bellringer
Although you may not be familiar with the specific details,
you have seen buoyant forces at work. You know from
experience that certain objects float in air or in water. This
is because of the force that pushes, or buoys the object up.
This force, which is always in the upward direction,
opposes the weight of the object.
Examine each of the drawings shown on the next slide.
Then answer the questions that follow.
States of Matter
Section 3
Bellringer, continued
1. Is the buoyant force on the lump of gold greater than, less than, or equal
to the gold’s weight?
2. Is the buoyant force on the balloon greater than, less than, or equal to the
balloon’s weight?
3. Is the buoyant force on the boat greater than, less than, or equal to the
boat’s weight?
4. Is the buoyant force on the submarine greater than, less than, or equal to
the submarine’s weight?
States of Matter
Section 3
Pressure
〉How do fluids exert pressure?
〉Fluids exert pressure evenly in all directions.
– pressure: the amount of force exerted per unit area
of a surface
– example: when you pump
up a bicycle tire, air particles
constantly push against each
other and against the tire walls
States of Matter
Section 3
Pressure, continued
• Pressure can be calculated by dividing force by the
area over which the force is exerted:
Force
F
Pressure 
, or P 
Area
A
• The SI unit for pressure is the pascal.
– pascal: the SI unit of pressure; equal to the force of
1 N exerted over an area of 1 m2 (symbol, Pa)
States of Matter
Section 3
Visual Concept: Equation for Pressure
States of Matter
Section 3
Buoyant Force
〉What force makes a rubber duck float in a
bathtub?
〉All fluids exert an upward buoyant force on
matter.
• buoyant force: the upward force that keeps an object
immersed in or floating on a fluid
States of Matter
Section 3
Buoyant Force, continued
• Archimedes’ principle is used to find buoyant force.
– The buoyant force on an object in a fluid is an upward
force equal to the weight of the fluid that the object
displaces.
States of Matter
Section 3
Comparing Weight and Buoyant Force
States of Matter
Section 3
Buoyant Force, continued
• An object will float or sink based on its
density.
– If an object is less dense than the fluid in which it
is placed, it will float.
– If an object is more dense than the fluid in which
it is placed, it will sink.
States of Matter
Density
Section 3
States of Matter
Section 3
Pascal’s Principle
〉 What happens when pressure in a fluid changes?
〉 Pascal’s principle states that a change in pressure at
any point in an enclosed fluid will be transmitted
equally to all parts of the fluid. In other words, if the
pressure in a container is increased at any point, the
pressure increases at all points by the same amount.
– Mathematically, Pascal’s principle is stated as
P1 = P2.
– Because P = F/A, Pascal’s principle can also be
expressed as F1/A1 = F2/A2.
States of Matter
Section 3
Pascal’s Principle, continued
• Hydraulic devices are based on Pascal’s principle.
– Because the pressure is the same on both sides of the
enclosed fluid, a small force on the smaller area (left) produces
a much larger force on the larger area (right).
– The plunger
travels through
a larger
distance on the
side that has
the smaller
area.
States of Matter
Section 3
Math Skills
Pascal’s Principle
A hydraulic lift uses Pascal’s principle to lift a 19,000 N
car. If the area of the small piston (A1) equals 10.5 cm2
and the area of the large piston (A2) equals 400 cm2,
what force needs to be exerted on the small piston to lift
the car?
1. List the given and unknown values.
Given:
F2 = 19,000 N
A1 = 10.5 cm2
A2 = 400 cm2
Unknown:F1
States of Matter
Section 3
Math Skills, continued
2. Start with Pascal’s principle, and substitute the
equation for pressure. Then, rearrange the equation
to isolate the unknown value.
P1 = P2
F1 F2

A1 A2
(F2 )( A1 )
F1 
A2
States of Matter
Section 3
Math Skills, continued
3. Insert the known values into the equation,
and solve.
(19,000 N)(10.5 cm2 )
F1 
400 cm2
F1 = 500 N
States of Matter
Section 3
Fluids in Motion
〉What affects the speed of a fluid in motion?
〉Fluids move faster through small areas than
through larger areas, if the overall flow rate
remains constant. Fluids also vary in the rate at
which they flow.
States of Matter
Section 3
Fluids in Motion, continued
• Viscosity depends on particle attraction.
– viscosity: the resistance of a gas or liquid to flow
• Fluid pressure decreases as speed increases.
– This is known as Bernoulli’s principle.
States of Matter
Section 3
Visual Concept: Viscosity