13.1 Fluid Pressure

13.1 Fluid Pressure • 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

13.1 Fluid Pressure