1. Chapter 11
Section 1 – pg 416
Pressure

2. Pg 416
What is Pressure?
Due to the force of gravity constantly pulling you down, your feet exert pressure on the ground

3. Pg 417
Pressure and Area
The amount of pressure you exert depends on the area over which you exert a force
The larger the area over which the force is distributed, the less pressure is exerted
This is why it takes so little pressure to pierce human skin with a needle, the surface area is very small

4. Pg 417
Calculating Pressure
Pressure is equal to the force exerted on a surface divided by the total area over which the force is exerted
Pressure = Force/Area
Measured in Pascals (newtons per square meter)
America uses psi (pounds per square inch)

5. Fluid Pressure Fluids also exert pressure Pg 418
Fluid: a material that can easily flow
Gases are considered a fluid
Water rushing from the hole at the bottom of the cup shoots out with greater force due to the fact that there is more pressure pushing it out

6. What Causes Fluid Pressure?
Pg 418
What Causes Fluid Pressure?
Particles in a fluid move constantly in all directions, while moving they collide with each other and any surface they meet.
As each particle collides with a surface, it exerts a force on the surface
In a fluid, all of the forces exerted by the individual particles combine to make up
the pressure exerted by the
fluid divided by the area over
which the force is exerted

7. Pg 418
Air Pressure
Gases in the atmosphere press down on everything on Earth’s surface, all the time
Air exerts pressure because it has mass
B/c the force of gravity pulls down on this mass of air, the air has weight. The weight of the air is the force that produces air pressure, or atmospheric pressure

8. Balanced Pressure
Pg 419
The weight supported by the surface area of your hand is about 1,000 Newtons (abt the same weight as a washing machine)
In a stationary fluid, pressure at a given point is exerted equally in all directions
Air Pressure presses on your hand
from all directions and these
pressures balance each other out
Your body also contains fluids that
exert an outward pressure

9. Variations in Fluid Pressure: Atmospheric Pressure and Elevation
Atmospheric pressure decreases as your elevation increases
At higher elevations, there is less air above you and therefore less air pressure
When the air pressure outside the body decreases, the air pressure inside also decreases but at a slower rate
Ears pop to balance pressures
Pg 421

10. Water Pressure and Depth
Pg 421
Water Pressure and Depth
Water pressure increases as depth increases due to increase amount of water above you
The deeper you swim, the greater the pressure you feel

11. Pg 422
Measuring Pressure
Barometer: instrument that measures atmospheric pressure
Used to forecast weather
Rapidly decreasing pressure means storm on way
Increase air pressure means fair weather

12. Chapter 11 Section 1 Homework Pg 422

13. 1A. What are two factors does pressure depend on?

14. 1B. Who exerts more pressure on the ground – a 500N woman standing in high heels with a total area of m2, or the same woman standing in work boots with a total area of 0.05 m2?

15. 2A. How do fluids exert pressure?

16. 2B. Why aren’t you crushed by the weight of the atmosphere?

17. 2C. How is your body similar to the can containing air shown in Fig. 4?

18. 3A. How does atmospheric pressure change as you move up away from sea level?

19. 3B. Compare the change in atmospheric pressure with elevation to the change in water pressure with depth

20. 3C. Why must an astronaut wear a pressurized suit in space?