Forces in Fluids
7:1 Fluids and Pressure
Fluid~ any material that can flow and takes the
shape of its container
Pressure~the amount of force exerted on a given
Pascal~SI unit for pressure
Atmospheric pressure~ pressure cause by the
weight of the atmosphere
Density~the amount of matter in a certain volume
Pascal’s Principle~ A change in pressure at any
point in an enclosed fluid will be transmitted
equally to all parts of that fluid
All Fluids Exert Pressure
•Pressure = Force ÷ Area
•1 N/m2=1 pascal
•Force of the air particles hitting the inner
surface of the tire creates pressure
•Why are bubbles round?
–fluids exert pressure evenly in all directions
–bubble expands in every direction
–helps to create a sphere
Atmospheric Pressure
• Atmospheric pressure is exerted on
everything on Earth
• Exerts approx. 101,300 N on every square
meter or (10 N per square centimeter)
• The fluids inside your body exert pressure
which works against atmospheric
• Atmospheric pressure varies
– Low at the top of atmosphere because there
is very little atmosphere to press down
Atmospheric Pressure (cont.’d)
• Pressure depends on depth
– Pressure decreases as you go higher into the
– Fluids in your body must adjust as you go
between higher and lower points
•Ears “popping”
•Air behind eardrum expanding or contracting
•“pop” due to air being released
Water Pressure
• Water pressure increases as depth
• Pressure depends on the depth of the
fluid not the total amount of fluid present
• Density makes a difference
– Water is more dense then air
– Water has a greater mass for a certain
– Waters exerts more pressure than air
Fluids Flow from High
Pressure to Low Pressure
•When drinking through a straw, you
remove air from inside the straw
•Pressure inside the straw is reduced
•Atmospheric pressure on surface of liquid
remains the same
•Pressure outside the straw is higher
pressure then inside the straw so the liquid
is pushed up the straw
Chapter 11 Section 3
Lets Review…
What is pressure?
What is the equation for pressure?
What causes fluid pressure?
When you catch a deep-sea fish, why does its eyes
Why do your ears pop on an airplane or up in the
Pascal’s Principle
When force is applied to a confined liquid, the
change in pressure is transmitted equally to
all parts of the fluid.
Draw a bottle of water with arrows to illustrate the
regular exerted pressure. Then draw a water
bottle that you squeeze. What happens to the
pressure? What happens if you open the top?
How does Pascal’s Principle explain what
happens if you squeeze a water bottle?
A force applied to one section of an
enclosed liquid at rest will be transferred
to the entire liquid with the same amount
of force.
Hydraulic Systems
A force applied to one piston increases the fluid
pressure throughout the fluid.
If the second piston has a larger surface area,
the force is multiplied!
Pressure = Force/Area
Force/Area = Pressure = Larger Force/Larger Area