Chapter 11 Section 1 – pg 416 Pressure

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

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

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)

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

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

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

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

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

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

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

Chapter 11 Section 1 Homework Pg 422

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

1B. Who exerts more pressure on the ground – a 500N woman standing in high heels with a total area of 0.005 m2, or the same woman standing in work boots with a total area of 0.05 m2?

2A. How do fluids exert pressure?

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

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

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

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

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