Pressure . Result of the force distributed over an areaP = F

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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

Pressure

Result of the force distributed over an area

P = F÷A

P = pressure (Pa)

F = force (N)

A = area (m2)

Pressure in a fluid

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

13.2 Forces and Pressure in Fluids

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

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

13-3 Buoyancy

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

Buoyancy

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