1. Forces in Fluids

2. Pressure

3. Pressure Pressure = Force in N F(N) Area in m2
A N/m2 is called a Pascal (Pa)
1000 Pa = 1 kPa
P (N/m2) A (m2)

4. Pressure What 2 things influence pressure?
If force is increased, pressure
increases
If area is increased, pressure
decreases
When do you exert more pressure on the floor - when standing on one foot or two feet?

5. Our Atmosphere Think about it: You live near the bottom of an
“ocean” of air
We don’t talk about depth of air like depth of water
Altitude – height above sea level
As altitude increases what happens to air pressure?
Air pressure decreases, because the
column of air above your head weighs less.
As you increase your altitude, your ears “pop” to equalize pressure.

6. Air Pressure Air is a fluid (mixture of gases) so it exerts a
pressure on objects
At sea level, 1 atmosphere is 14.7 psi or 101,300 Pa
Why don’t we feel this?
Pressure inside our bodies balances the pressure outside
Therefore, the net force is zero and we don’t get crushed
What happens when the pressures are not equal?

7. Fluid Pressure Fluid – substance that does not have a fixed shape.
Liquids and gases are fluids.
Fluids exert pressure on the substances within them.

8. Fluid Pressure Fluid pressure is often measured using units:
Atmospheres (atm)
Millimeters of mercury (mm Hg)
Pounds per square inch (psi)

9. Going Deeper In Water Have you ever dove deep
into a pool, lake, or ocean?
How does it make
your ears feel?
Gases can be compressed,
or squeezed into a smaller space.

10. How does depth affect pressure?
The greater the depth within a fluid the greater the pressure.
The deeper you go, the greater the weight of the column of fluid above you and therefore a greater force over the same size area.
The pressure in a fluid at any given depth is constant and exerted equally in all directions.
The 2 things that determine pressure exerted by a fluid that is not moving are DEPTH and TYPE OF FLUID.

11. Check your understanding
Where is the pressure greater: 3 m deep in the ocean or 8 m deep in the ocean?
Where is the pressure greater: 25 cm deep
in water in a bathtub or 25 cm deep in
water in a lake?
Where is pressure greater: 3 feet deep in water or 3 feet deep in air?

12. Rising to the Surface – The “Bends”
Under high pressure, higher concentration of
gases dissolved in blood.
As you rise to the surface, pressure
decreases
If ascent is made too rapidly, dissolved gases form bubbles that can migrate to various parts of body, especially noted in the joints.
This causes Decompression Sickness, or the “Bends” - pain and various other ailments