Forces in Fluidsp. 92-98

Pressure

• Since fluids take different shapes, the concept of a force on a fluid is better described as PRESSURE. You can’t really “push” a fluid but you can exert pressure on it.

• Pressure is the amount of force applied to a fluid per unit surface area– Bike tire has 100 psi (pounds per square inch)– Car tire has 35 psi (pounds per square inch)

Formula to calculate pressure

Pressure (Pa) = Force (N) area (m2)

OR

P = F/a

P is pressure measured in Pascals (Pa)F is force exerted measured in Newtons (N)a is surface area measured in m2

Pressure in a Liquid

• In a liquid, pressure depends on the density of the liquid and the depth– Higher the density the

higher the pressure– Increased depth means

increased pressure– SCUBA divers

experience increased pressure the deeper they dive….you can feel it in your ears!!!

Pressure in a Gas

• In a gas, pressure depends on the temperature, volume and number of gas particles

• Higher temperature means higher pressure because the molecules are moving faster

• Increasing the volume results in lowering the pressure because there is more room for the molecules to move

• Increasing the number of gas molecules will increase the pressure

As volume increases, pressure decreases

Pascal’s Principle

• An increase in the pressure on a fluid is transmitted uniformly in all directions

• Used in water pistols, hydraulic breaks, syringes.

• See Fig. 3.33 on p. 94 (hydraulic lift)

Hydraulic breaks and jacks use Pascal’s principal

Archemedes’ Principle or “Why an 80 000 ton battleship can float!”

Which will float?

Archimedes’ Principle

• Since the pressure in a liquid increases with depth, an upward force called “buoyancy” is exerted on objects placed in liquids

• Archimedes’ Principle: the magnitude of the buoyant force will equal the weight of the fluid displaced by the object

• Archimedes’ Principle explains why some objects will float in water and some will sink.

• It also explains why objects feel “lighter” when underwater.

• Why does a steel ship float in water?• It all has to do with the shape of the hull

and the weight of water displaced by the ship’s hull.

Archemedes’ Principle

Archimedes’ Principle

1. If the buoyant force is less than the force of gravity (Fb<Fg) the object will sink

2. If the buoyant force is greater than the force of gravity (Fb>Fg) the object will float

3. If the buoyant force is equal to the force of gravity (Fb=Fg) the object will maintain the same depth

How a submarine works

• See also Fig. 3.37, p. 97

How a submarine works

Bernoulli’s Principle or how a 875 000 pound airplane can fly

Bernoulli’s Principle

• When a fluid or gas is in motion, like the wind in the air, its pressure varies with its speed

• Bernoulli’s Principle: the higher the speed of the fluid or gas, the lower its pressure

• This explains how a plane can fly (Fig. 3.38, p. 98)

Bernoulli’s Principle

When the force from the lift exceeds that of gravity, the plane rises into the air.