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13.1 Fluid Pressure

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- After completing this section, you will be able to:
- Describe and calculate pressure
- Identify appropriate SI units for measuring pressure
- Describe the relationship between water depth and the pressure it exerts
- Describe how forces from pressure are distributed at a given level in a fluid
- Explain how altitude affects air pressure

- Result of the force distributed over an area
- P = F÷A
- P = pressure (Pa)
- F = force (N)
- A = area (m2)

- Fluid is a substance that assumes the shape of its container
- As the depth of a fluid increases, the pressure increases
- At a particular depth, the pressure in a fluid is constant and exerted equally in all directions

- Shape of the container and the area of the container’s bottom do not affect the fluid pressure
- Different fluids exert different pressures
- Air pressure is a type of fluid pressure
- As altitude increases, air pressure decreases

- After completing this section, you will be able to:
- Describe how pressure is transmitted in a fluid according to Pascal’s principle
- Explain how the speed and pressure of a fluid are related according to Bernoulli’s principle

- Fluids exert pressure equally in all directions at a given depth
- Amount of pressure depends on the type of fluid and the fluid depth

- Any change in pressure at any point in a fluid is transmitted equally and unchanged in all directions
- Called Pascal’s principle
- Example: when you squeeze a bottle filled with water, the pressure change is equally spread throughout the whole bottle
- Application of Pascal’s principle is a hydraulic system
- Device that used pressurized fluid acting on pistons of different sizes to change a force

- The speed of a fluid can change the pressure of a fluid
- As the speed of a fluid increases, the pressure within the fluid decreases
- Called Bernoulli’s principle
- The faster the fluid, the lower the pressure

- As the speed of a fluid increases, the pressure within the fluid decreases

- Application of Bernoulli’s principle is the wings of planes and birds
- Pressure difference created by the fluid moving at different speeds causes an upward force called lift

- Spoilers on cars are upside down wings
- Spray bottles

- After completing this section, you will be able to:
- Explain the effect of buoyancy on the apparent weight of an object
- Explain the relationship between the volume of fluid displaced by an object and buoyant force acting on the object according to Archimedes’ principle

- Describe the relationship among object density, fluid density, and whether an object sinks or floats in a fluid
- Describe the relationship among object weight, buoyant force, and whether an object sinks or floats in a fluid

- Ability of a fluid to exert an upward force on any object placed in it
- Results in the apparent loss of weight of an object in a fluid
- Called the apparent weight

- Upward force that acts opposite of gravity is buoyant force

- Since water pressure increases with depth, the forces pushing up on the bottom of an object are greater than the buoyant forces pushing down on the top of an object
- Buoyant force on an object is equal to the weight of the fluid displaced by the object
- Called Archimedes’ principle

- Buoyancy is closely related to density
- If an object is less dense than the fluid it is in, it will float
- If an object is more dense than the fluid it is in, it will sink

- When the buoyant force is greater or equal to the weight, an object will float
- When the buoyant force is exactly equal to the weight, an object is suspended
- Floats at any level in the fluid

- When the buoyant force is less than the weight, an object will sink

- Why does a clay block sink while a clay boat floats?
- The shape of the boat allows it to displace a larger volume of water relative to its weight
- The heavier the boat, the more water it must displace in order to float
- The larger the boat, the more volume it has and the less its density