Chapter 3: Forces in Fluids
Pressure
What is Pressure?
Pressure refers to a force pushing on a
surface.
What are some examples of pressure?
1. Leaning against a wall
2. Standing on the ground
Forces and Pressure
Why does the woman wearing snow shoes
stay on top of the snow and the two girls sink
into the snow?
Forces and Pressure
The downward force, your weight, exerted on the
snow does not change, so what is the
difference?
The difference is the size of the area over which
your weight is distributed.
When your weight is distributed over the larger
area of the snowshoes, there is less downward
pressure on the snow under the shoes.
Calculating Pressure
Pressure = Force
Area
Force is measured in Newtons
Area is measured in m2
Pressure is measured in N/m2
The unit for pressure is also called a Pascal
1 N/m2 = 1 Pa
Fluid Pressure
A fluid is a substance that can easily flow.
A fluid is able to change shape, therefore
liquids and gases are both fluids.
Fluids
Fluids exert pressure against the surfaces they
touch. In fluids, molecules are constantly
moving in all directions and colliding with
each other and any surface they meet.
Fluid Pressure
As each molecule collides with a surface, it
exerts a force on the surface.
All of the forces exerted by the individual
molecules in a fluid add together to make up
the pressure exerted by the fluid.
Fluid pressure is the total force exerted by the
fluid divided over the area over which the
force is exerted.
Fluid Pressure All Around
Pressure exerted by the air is
called air pressure or
atmospheric pressure.
Air exerts pressure because it
has mass. (About 1 kg for
every 1 m3)
The force of gravity on the
mass of air produces an
average air pressure (at
sea level) of 10.13 N/cm2.
Balanced Pressures
Why doesn’t the pressure of the
air around you crush you?
The reason is because the
pressure inside your body
balances the pressure outside
of your body.
Balanced Pressures
Some parts of your body, such as your lungs,
sinus cavities, and inner ear, contain air.
Other parts of your body, such as your cells
and blood vessels, contain liquids.
Variations in Fluid Pressure
Have you ever had your ears “pop” when you
drive up the side of a mountain? Why?
Air pressure decreases as elevation increases.
Variations in Fluid Pressure
When the air pressure outside of your body
changes, the air pressure inside will adjust
too, but more slowly. For a moment, the air
pressure behind your eardrums is greater
than it is outside. Your body releases this
pressure with a pop so that the pressures are
once again balanced.
Variations in Fluid Pressure
What do you feel when you swim down to the
bottom of a deep pool or lake? Why?
Water pressure increases as depth increases.
Variations in Fluid Pressure
The water pressure increases as you go down
because there is more water above you. In
addition, air in the atmosphere is pushing
down on the water. The total pressure below
the water results from the weight of the water
and air above you.