Properties of FluidsSolids, Liquids, and Gases, Section 2

Buoyancy

Buoyancy is the ability of a fluid to exert an upward force on an object immersed in it.!

If the buoyant force is equal to the weight of the object, the object will float.

Archimede’s Principle

Archimedes was a Greek mathematician who lived in the third century B.C. !

He discovered that the buoyant force on an object equals the weight of the fluid displaced by the object. If the weight of the water displaced equals the weight of the object, the object will float.

Density

If an object floats, the density of the object is less than the density of the water.!

The combination of the steel hull and the air has a lesser density than that of water.

Pascal’s Principle

If you swim underwater, you feel pressure all around you, especially if you swim in the deep end in a pool.!

Pressure is force per unit area or P = F / A!

Blaise Pascal, a French scientist, discovered that pressure is transmitted through a fluid (fluids are liquids and gases).

Applying Pascal’s Principle

Hydraulic machines use Pascal’s principle to move heavy loads, such as in a front end loader

Applying Pascal’s Principle (cont.)

Pressure is applied to one cylinder and is transmitted through a fluid (oil) to another cylinder which is used to move a load.!

The pressures in both cylinders are the same.

Calculation Forces #Using Pascal’s Principle

Since the pressure in the first cylinder equals the pressure in the second cylinder, P1 =P2 so F1/A1 = F2/A2!

You need to know three of the four variables to solve the problem!

Example: A hydraulic lift is used to lift a car that is pushing down with a force of 13,000 newtons on a cylinder with an area of 1.56 m2. What force is needed to push down on a cylinder of 0.015 m2?

Calculation Forces Using#Pascal’s Principle (Continued)

First: List what you know!

F1 = 13,000 newtons, A1 = 1.56 m2. !

F2= ?, A2 = 0.015 m2!

Second: Decide what equation you want to use. !

F1/A1 = F2/A2

Calculation Forces Using#Pascal’s Principle (Continued)

Third: Decide what variable you need to solve for (F1/A1 = F2/A2), and use algebra to see up the new equation.!

F1/A1 = F2/A2!

We need to solve for F2, so multiple both sides by A2 to cancel out A2 on the right. F1A2/A1 = F2A2/A2

Calculation Forces Using#Pascal’s Principle (Continued)

Fourth: Plug the numbers into for the variables, and solve for the unknown. !

F2 = F1A2/A1!

F2 = (13,000 N) (0.015 m2) / 1.56 m2!

F2 = 125 N, Remember to circle your answer and show all of your work!

Bernoulli’s Principle

Daniel Bernoulli was a Swiss scientist who studied the properties of moving fluids.!

According to Bernoulli’s principle, as the speed of a fluid increases, the pressure by the fluid decreases.!

His principle helps explains how an airplane wing works.

Bernoulli’s Principle (Continued)

Bernoulli’s Principle (Continued)

Fluid Flow

Another property exhibited by fluid is its tendency to flow.!

The resistance of a fluid to flow is called viscosity.!

If a liquid flows easily, it has a low viscosity, it has a resistance to flowing (such as thick syrup) it has high viscosity.