Forces in Fluids

Pressure, Floating/Sinking, Pascal’s Principal

&Bernoulli’s Principal

Pressure (1)

Pressure relates to anything exerting force on another object by pressing on it.

Pressure = Force/Area

Units will be in Newton per squared meter: Nm2

Pressure (2)

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

Examples: – Snow shoes vs. regular shoes– Light pressure used with needles for shots– High heels in grass vs. regular shoes

Fluid Pressure (1)

Remember back to fluids… Fluids are a material that can easily

flow and change shape. If you understand the pressure in

fluids, you can predict whether an object will float or sink.

**** Fluids are not just liquids!! They can also be gasses- look at the definition!!!

Fluid Pressure (2)

In a fluid, all of the forces exerted by the individual particles combine to make up the pressure exerted by the fluid. – The denser the fluid, the more pressure it

will exert (because there are more particles to exert pressure).

Fluid Pressure (3)

Air pressure is also considered to be a type of fluid pressure.

We are constantly surrounded by this pressure of the air around us.

Although it changes at different elevations, our bodies are designed to withstand it without it crushing us. – Sea Level atmospheric pressure is a

whopping 101,300 Pa!!!!! (Pascal units)

Measuring Pressure

We use Barometers to measure atmospheric pressure.

People to make weather predictions use Barometers to help understand weather patterns and make forcasts.

Floating and Sinking (1)

Remember Density? Mass per unit volume???

We can calculate density by dividing mass by volume!

Density = Mass / volume

Floating and Sinking (2)

If you know the densities of objects you can predict whether they will sink or float when placed in a fluid of a given density.

Remember if the density is larger than the fluid it’s placed in it will sink, if it’s smaller it will float. (let’s see what you’ve learned…)

Floating and Sinking (3)

If I had water (1g/cm3) would the following float or sink?– Wood (.7 g/cm3 )– Tar (1.02 g/cm3 )– Corn Oil (.925 g/cm3 )– Mercury (13.6 g/cm3 )– Corn Syrup (1.38 g/cm3 )

Buoyancy (1)

Buoyancy is the ability to float. Water and other fluids exert an upward

force on any object submerged into it. Buoyancy acts in the opposite direction

of gravity, so it makes an object feel lighter. – Ever notice how it’s much easier to lift or

carry a friend in a pool than it is outside the water?

Buoyancy (2)

Weight of submerged objects is a downward force and the buoyancy of an object is an upward force.

If an objects weight is greater than the buoyant force it will sink.

If an objects weight is equal to the buoyant force it will not sink.

Buoyancy (3)

Pressure gets greater, the lower you are in a fluid. – You have the weight of all of the fluid on

top of you and the lower you get, the more weight is on top of you.

– It’s like playing “dog pile”. You never want to be on the bottom or you get crushed!

Archimedes Principal (1)

Archimedes Principal states that the buoyant force acting on a submerged object is equal to the weight of the volume of the fluid displaced by the object.

Meaning, if you hand displaces 50 ml of water when I stick in a cup (stupid, I know) then the buoyant force acting on it would be equal to the weight of 50ml.

Archimedes Principal (2)

So how does this apply to ships? If the buoyant force of an object is

equal to the weight of the displaced fluid, then a large fluid that displaces a lot of fluid should have a strong buoyant force.

If you are confused, check out the picture on page 429 in your text book.

Pascal’s Principal (1)

Fluids exert pressure on any surface that it touches.

Pascal’s Principal states that when a force is applied to a confined fluid, the change in pressure is transmitted equally to all parts of the fluid. – An example of Pascal’s principal at work

is hydraulic pistons.

Pascal’s Principal (2)

Hydraulic systems multiply forces by applying the force to a small surface area. The increase in pressure is then transmitted to another part of the confined fluid, which pushes on a larger surface area. – Hydraulic lifts– Hydraulic breaks

Bernoulli’s Principal (1)

Fluids (remember, not just liquids) flow from areas of high pressure to low pressure. This is an attempt to keep things balanced.

The pressure of a moving fluid is different than the pressure of a fluid at rest (not moving).

Bernoulli’s Principal (2)

Bernoulli’s principal states that the faster a fluid moves, the less pressure the fluid exerts. – Therefore, as the speed increases, the

pressure exerted by the fluid decreases.

Bernoulli’s Principal (3)

This principal is used to explain the flight of objects (kites, airplane, frisbee etc).

If the air moves faster above the object, fluid pressure pushes the object upward. – The air moves faster due to the

shape/design of the object (typically a curved shape with a slightly rounded top)