Pressure A force distributed over a given area Equation for Pressure: Pressure = Force / Area Force = Newton’s Area = m2

Units for Pressure: Pascal (Pa)

Forces in Fluids

A woman’s high heels sink into the soft ground, but the larger shoes of the much bigger man do not. Why???

Pressure = Force / Area More area = less pressure

Forces in Fluids

Example #1 Each tile on the bottom of a swimming pool has an area of 0.50 m2. The water above each tile exerts a force of 11,000 N on each tile. How much pressure does the water exert on each tile?

P = F / A P = 11,000 N / 0.50 m2

P = 22,000 Pa

Forces in Fluids

Example #2 The weight of a gallon of milk is about 38 N. If you pour a gallon of milk into a container whose bottom has an area of 0.60 m2, how much pressure will the milk exert on the bottom of the container?

P = F / A P = 38 N / 0.60 m2

P = 63.33 Pa

Forces in Fluids

What is a Fluid? A substance that attains the shape of its container. Examples: liquids and gases. §  Fluid pressure changes with depth in a column of water

and with altitude.

§  Fluid pressure in water does not depend on the volume or weight. It depends only on the depth.

Forces in Fluids

Two factors that affect fluid pressure: 1.  Depth 2.  Type of fluid

Forces in Fluids

Fluid Pressure – In the Water

How does air pressure change as altitude changes? Air pressure decreases as the altitude increases. Examples: •  Ears popped while on a plane •  Driving up mountains

Fluid Pressure – In the Air

§  Blaise Pascal 1623-1662 §  French physicist and mathematician §  Performed some of the first

experiments dealing with pressure

Forces in Fluids

Pascal’s Principle A change in pressure at any point in a fluid is transmitted equally and unchanged in all directions throughout the fluid.

Pascal’s Principle

A force applied to one section of an enclosed liquid at rest will be transferred to the entire liquid with the same amount of force.

Pascal’s Principle

Pascal’s Principle Examples

Forces in Fluids

14.2 Assessment

What is a hydraulic system? A device that uses pressurized fluid acting on pistons of different sizes to change a force. •  If the second piston has a larger surface area, the force is

multiplied!

•  Pressure = Force/Area

•  Force/Area = Pressure = Larger Force/Larger Area

Forces in Fluids

Force is applied to the piston Force is transmitted

through the fluid to the other piston

On the larger piston is multiplied

Forces in Fluids

Many everyday items uses Pascal’s principle and moving pistons with an enclosed pressurized fluid.

Forces in Fluids

Everyday Examples §  Car brakes §  Amusement park rides

Forces in Fluids

Everyday Examples §  Car jacks §  Construction machines

Forces in Fluids

Everyday Examples §  Car lifts §  Office chairs §  Hollywood????

Forces in Fluids

Bernoulli’s Principle As the speed of a fluid increases, the pressure within the fluid decreases. Air always moves from higher pressure to lower pressure

Forces in Fluids

How do we use Bernoulli’s principle to make planes fly?

Forces in Fluids

How do we use Bernoulli’s principle to make planes fly (Draw diagram)?

Forces in Fluids

Forces in Fluids

Two natural forces being exerted on plane:

Weight Downward force of gravity Drag Resistance force created by the plane’s movement through the air

Weight

Drag

Two forces a pilot needs to create to overcome weight and drag

Lift Upward force on a plane (Due to force on wings) Thrust Artificially created (engines). Used to overcome drag and to sustain lift

Lift

Thrust

14.2 Assessment

1.  What two things must you know to calculate pressure?

Force and Area

14.2 Assessment

2.  What is the relationship between the depth of water and the pressure it exerts?

Water pressure increases as depth increases

14.2 Assessment

3.  How is the pressure distributed at a given depth in a fluid?

At any given depth, pressure is constant and exerted equally in all directions.

14.2 Assessment

4.  How does the pressure exerted by the atmosphere change as altitude increases?

Pressure decreases as altitude increases

14.2 Assessment

5.  Give me two examples of Pascal’s Principle

14.2 Assessment

6.  How are fluid speed and fluid pressure related.

As the speed of a fluid increases, the pressure within the fluid decreases.

14.2 Assessment

7.  Whose Principle is this?

Bernoulli’s Principle

14.2 Assessment

Problem #8: The weight of the gasoline in a 55 gallon drum creates a force of 1,456 Newton’s. The area of the bottom of the drum is 0.80 m2. How much pressure does the gasoline exert on the bottom of the drum?

1,820 Pa

14.2 Assessment

Problem #9: A company makes garden statues by pouring concrete into a mold. The amount used to make a statue if a deer weighs 3,600 N. If the base of the deer statue is 0.60 m long and 0.40 meters wide, how much pressure will the statue exert on a the ground? (Hint: Area is equal to length times width)

15,000 Pa

14.2 Assessment

Problem #10: A book with a weight of 12 N rests on its back cover. If the books cover measures 0.21 m by 0.28 m, how much pressure does the book exert?

204.08 Pa