Properties of Fluids
Some ships are so huge that they are like floating cities. For example, aircraft carriers are so large enough to allow airplanes to take off and land on their decks.
The supporting force is called the buoyant force. Buoyancy is the ability of a fluid or gas to exert an upward force on an object immersed in it.
If the buoyant force is equal to the object’s weight, the object will float, but if the buoyant force is less than the object’s weight the object will sink.
Archimedes’ Principle: “Eureka, I found it!!!!”
Archimedes found that the buoyant force on an object is equal to the weight of the fluid displaced by the object.
Example: When you get into a bath, does the water rise or stay the same? Of course the water rises. Where else does your dirt go?
Density:
What would two equal sized but different material objects do when placed in water, such as a piece of wood and a piece of steel? Would both float or sink?
Of course the steel would sink and the wood would float, but why?
The answer lies with density.
Density = Mass/Volume
If the wood is less dense than the water, the piece of wood will float, but if the steel is more dense than the water, the piece of steel will sink.
Pascal’s Principle
If you are underwater, you can feel the pressure of the water all around you. Pressure is force exerted per unit area.
The atmosphere exerts pressure all around you.
Applying the Principle:
Hydraulic machines mare machines that move heavy loadsin accordance with Pascal’s Principle.
Formula: pressure = Force/Area
Or
Pressure = F/A
You can also use:
F/A (1)= F/A (2)
Problem:
A hydraulic lift is used to lift a heavy machine that is pushing down on a 2.8 m² piston (A1) with a force (F1) of 3,700 N. What force (F2) needs to be exerted on a .072 m² piston (A2) to lift the machine?
A1 = 2.8 m²
F1 = 3,700 N
A2 = .072 m²
F2 = ?
(F1)(A2) / (A1)
F2 = (3,700 X .072) / 2.8
F2 = 95 N
Bernoulli’s Principle
Why do planes fly, but racecars do not?
According to Bernoulli’s principle, as the velocity of a fluid increases, the pressure exerted by the fluid decreases.
One way to demonstrate Bernoulli’s principle is to blow across the top surface of as sheet of paper. The paper will rise; the velocity of the air you blew over the top surface of the paper is greater than that of the quiet air below it.
An airplane wing is designed to be flat on the bottom and curved over the top. This allows the high pressure air to pass below the flat portion of the wing (bottom), and have the low pressure air flow above it which will cause lift.
Which way do racecars have their “wings”?
Another property exhibited by fluid is its tendency to flow. A resistance to flow by a fluid is called viscosity.
If it moves slow, it has High viscosity, and if it move quickly, it has low viscosity.
Which kind do you want to have in your cars?