Forces In Fluids

• Fluid Pressure• Hydraulic Devices• Pressure and Gravity• Buoyancy• Fluids in Motion

Fluid Pressure

• Pressure is defined as the force that particles exert over a certain area due to their weight and motion.

• Pressure=Force/Area• N/cm2

• Fluids move from areas of higher pressure to areas of lower pressure.

• All liquids and gases are fluids.

Pressure (continued)

Pressure (continued)

Pressure (continued)

Pressure (continued)

Pressure (continued)

The greater the density of gas particles, the greater the pressure they exert.

A B

Hydraulic Devices

• Pressure in a fluid is the same throughout and is exerted equally in all directions when there are no outside forces acting on the fluid.

• Hydraulic devices are machines that take advantage of the fact that pressure is transmitted equally in all directions in a liquid.

Hydraulic Devices (continued)

• Hydraulic devices apply small force to a small area to develop a large net force over a larger area.

• Examples include car brakes, barber shop chairs, and snow plows.

Hydraulic Devices (continued)

Examples of hydraulic devices

Pressure and Gravity

• Due to the force of gravity the pressure of any fluid increases with depth.

• Air pressure decreases as altitude increases. This is called an inverse relationship.

Pressure and Gravity (continued)

Pressure and Gravity (continued)

Buoyancy

• Buoyancy is defined as the upwards force in a fluid which acts opposite of gravity.

• The buoyant force on an object is equal to the weight of the fluid displaced by the object.

• This concept is known as Archimedes’ Principle.

Buoyancy (continued)

Archimedes and the Case of the Golden Crown I n the fi rst century BC, a Roman architect named Vitruvius told a story of how Archimedes uncovered a f raud in the making of a golden crown made f or Hiero I I , the king of Syracuse. Suspecting that the goldsmith might have replaced some of the gold given to him with an equal weight of other, less valuable metals, Hiero asked Archimedes to determine whether the crown was pure gold. Since the crown was considered a holy object dedicated to the gods, Archimedes could not disturb or destroy the crown in any way. To determine what the crown was made out of , Archimedes knew that he would need to work out the volume of an irregular solid. He thought about this f or many days, but he had trouble thinking of a reliable way to approach this dilemma. The solution fi nally occurred to him when he was visiting the public baths. When he stepped into the water he noticed how the water overflowed the sides of the bath. He realized that the volume of the spilt water was equal to his bulk. He now knew how to fi nd the volume of an irregularly shaped object like a human body or a king’s crown! Archimedes jumped out the bath and ran naked through the streets of Syracuse shouting “Eureka, eureka!”

Buoyancy (continued)

Buoyancy (continued)

• An object floats when it displaces a volume of fluid whose weight is greater than or equal to its own weight.

• Density is the ratio of the mass of a substance to its volume.

• Density= Mass/Volume• g/cm3 or g/ml• An object is neutrally buoyant if its weight

and buoyant force are equal.

Buoyancy (continued)

• An object will float in a fluid if its density is less than the density of the fluid.

• Water has a density of 1g/cm3.• The densities of most woods vary

from .3 to .9g/cm3.• Gold has a density of 19.3g/cm3.

Buoyancy (continued)

Buoyancy (continued)

Buoyancy (continued)

Bernoulli’s Principle

• Bernoulli’s Principle explains that the pressure in a moving stream of fluid is less than the pressure in the surrounding fluid.

• The faster a fluid moves, the less pressure it exerts.

• This is the reason airplanes can fly.

Bernoulli’s Principle