**Forces in Fluids**

**By: Mr. Begoyan**

**Page 45 in ISN (page 416-441 in textbook)**

**Pressure**

Pressure

(symbol:

p

or

P

) is the force applied perpendicular to the surface of an object per unit area over which that force is distributed.

The amount of pressure depends on the area on which you exert the pressure.

Example: Calculate the pressure of 300N being applied to an area that is 10cm and an area that is 100 cm . Compare the two pressures.

2

2

The SI unit for pressure is the

pascal.

A

fluid

is a substance, a liquid or a gas, that is capable of flowing and that changes its shape at a steady rate when acted upon by a force.

Fluid Pressure

Fluid pressure

depends on pressure exerted by the height of a column of fluid and the density of that fluid.

Density of column

Height of column

x

Fluid Pressure

=

Fluids are made of tiny particles, each constantly in motion and exerting a force on it's surroundings.

In water, as the depth increases, so does the height of the column of water, increasing the fluid pressure.

Air or atmospheric pressure is also a result of a column (of air) pressing down on an area.

As the elevation increases, the number of air particles decreases (shorter column), decreasing air pressure.

Most objects exert enough internal pressure to counter external air pressure.

If, however, you remove the air from an object, the pressure from outside will meet no resistance from the inside.

Net force = 0

Barometers

are used to measure air pressure.

**Density and Buoyancy**

Density

is the measure of how much matter is squeezed into a given space.

Density

=

Mass

/

Volume

Both of the cubes occupy a space that is 1 cm.

The copper has a mass of 8.8 grams.

The plastic has a mass of 0.93 grams.

There is more copper matter in the same amount of space, thus copper is more dense.

3

By comparing densities, you can tell if an object will float or sink.

If an object is less dense than its surroundings, it will float, and vice versa.

Submarines and certain fish change their density in order to dive or surface.

Water and other fluids exert an upward force on submerged objects called the

buoyant force

.

The buoyant force acts in the opposite direction to gravity, thus making objects feel lighter.

Archimedes' principle

states the buoyant force is equal to the amount of water displaced by the submerged object.

If the object's mass is less than the mass of water it displaces, it will float.

Archimedes

Pascal's and Bernoulli's Principles

Any externally applied pressure is transmitted to all parts of the enclosed fluid, making it possible to amplify the force.

Applying a force to a small area, and transmitting it to a larger area using an enclosed liquid, will multiply the force.

This is the basis for all

hydraulic systems

.

Pascal's principle

states that pressure exerted anywhere in a confined incompressible fluid is transmitted equally in all directions throughout the fluid.

Bernoulli's principle

states that as the speed of a moving fluid increases, the pressure exerted by the fluid decreases.

Air has to speed up to go over a wing, decreasing its pressure.

This generates

lift

(an upwards force), and is the basis for all flight.

Air speeds up right here

Question set 1

1. How would you define pressure?

2. What is the unit for pressure?

3. How is pressure measured?

4. What is a fluid?

5. How does fluid pressure work?

6. Is fluid pressure different from air pressure?

7. If you take a sealed container to a sufficient depth, it would be crushed, if you take the same container into outer space, it would explode, why?

Question set 2

1. What is density?

2. What can we do with this knowledge (about density)?

3. What is buoyant force?

4. What does Archimedes' principle state?

5. What does Pascals' principle state, and what are some its applications?

Lift

6. What does Bernoulli's principle state, and what are some it's applications?

Homework

On page 44 of your ISN describe the following terms to a 5th grade audience using words and visuals:

Density, Pressure, Buoyancy, Lift.