Chapter 23

Forces in Fluids

Why is the electricity produced at the bottom of dams?

When you catch a deep-sea fish, why does its eyes pop-out?

Why do your ears pop on an airplane or up in the mountains?

Thought Questions

Pressure• Pressure is equal to the force applied

to a surface, divided by the area.

                                                                            

Equations for Pressure• Pressure = Force/surface area

•Pressure = Newtons (Kg x m/s2)side x side

•Units are in Pascals or N/m²

• A substance that can easily change its shape, such as liquids and gases.

• The molecules in a fluid have a certain amount of force (mass and acceleration) and exert pressure on surfaces they touch.

Fluids

•All the molecules add up together to make up the force exerted by the fluid.

High

High Pressure

Microburst - High Pressure Air

Low Pressure

L

L

Rising AirSuction

•Air has a mass of 1Kg/m³

•Gravity creates an air pressure of 10.13N/m³ at sea level.

• Air Pressure decreases as elevation increases.

Pressure and Elevation

Climbers above 20,000 feet need oxygen because the air is so thin

• Air Pressure decreases as elevation increases.• Astronomical telescopes are placed on mountain tops because

the thinner air means less interference with the stars

Pressure and Elevation

Mauna Kea at 13,700 feetIs above 40% of the atmosphere

Mount Everest at 29,600 feetIs above 70% of the atmosphere

Extreme Elevations

Commercial flights fly higher than you can breathe. So, they

must pressurize the cabin

35,000 feet

Extreme Elevations

Astronauts have no actually left the atmosphere. The microscopically thin atmosphere results in drag on spacecraft and satellites.

Nonetheless, it is far too thin to breathe and cold to survive without spacesuits

100 km

Very Low pressure

Higher Pressure

The whole system is a low pressure, but it is

extremely low in the eye.

Highest Pressure

Very Low pressure

Higher Pressure

Pressure always flows from high to low, which creates the high velocity winds toward the eye.

Highest Pressure

Very Low pressure

Higher Pressure

The Low Pressure creates a vacuum that pulls water up

into a dome. The high winds push this dome on shore.

Storm Surge

Very Low pressure

Higher Pressure

The force of the air moving toward the eye causes extreme winds in

excess of 100 mph.

Storm surges are the leading cause of damage

•Water pressure increases with depth.

Water PressureMarianas Trench = 10,911 m (35,800 ft) or 1,100 atmospheres.

That is the equivalent of an average-sized person holding up 48 jumbo jets or 400 times the pressure in automobile tires.

• When a force is applied to a confined fluid, the increase in pressure is transmitted equally to all parts of the fluid.

• These styroform cups were crushed outside of submarines.

Transmitting Pressure in a Fluid• When force is applied to a confined fluid, the change in

pressure is transmitted equally to all parts of the fluid.

Hydraulic Devices

• In a hydraulic device, a force applied to one piston increases the fluid pressure equally throughout the fluid.

Hydraulic Devices

• By changing the size of the pistons, the force can be multiplied.

Hydraulic Brakes

• The hydraulic brake system of a car multiplies the force exerted on the brake pedal.

The tendency or ability of an object to float.

Buoyancy• The pressure on the bottom

of a submerged object is greater than the pressure on the top. The result is a net force in the upward direction.

Buoyant Force

The upward force exerted by a fluid on a submerged or floating object.

Buoyancy• The buoyant force works opposite the

weight of an object.

Archimedes’ principle:• Buoyant Force on an object immersed in a liquid

equals the weight of the liquid displaced.

Density and buoyancy: An object that has a greater density than the fluid it is in, will sink. If its density is less than the fluid it will float.

It does not matter that Mercury is a liquid. It is so dense, most things float on it

Buoyancy of gases in liquids

Buoyancy of gases in gases

Gases have less density than water, so they rise up to the surface

Some gases, like Helium, are lighter than air and rise.

Some gases, like carbon dioxide, are heavier than air and sink.

How do super heavy container ships float?

• A solid block of steel sinks in water. • A steel ship with the same mass floats on the surface.

• A solid block of steel sinks in water. A steel ship with the same mass floats on the surface.

Air

steel

• Take the mass of the displaced area (steel and air) and compare it to the mass of the water of that volume.

• If the displaced area weighs less it will float.

Water

Density• Changes in density cause a submarine to

dive, rise, or float.

Density• Changes in density cause a submarine to

dive, rise, or float.

Density• Changes in density cause a submarine to

dive, rise, or float.

Density• Gases can actually be made from solids using

chemical reactions to create additional buoyancy.

1g/cm³or 1 g/ml