FORCES IN FLUIDS CHAPTER 11

Section 11-1 Pressure Pressure - related to the word press - refers to the force pushing on a surface

Force & Pressure The size of an area over which pressure is exerted makes a difference. No snowshoes Snowshoes

Calculating Pressure This formula summarizes the relationship between pressure, force, & area. Force is measured in N - newtons Area is measured in square units (meters)

Pascals So pressure is measured in the unit called the pascal (Pa): 1 N/m2 = 1 Pa

Force in a Fluid The diagram shows that the fluid force on 1 square cm is 12 N. What is the pressure? 12N/1cm = 12 N/cm2

What would happen if you made the area affected by the force smaller or larger?

You can produce a lower pressure by increasing the area a force acts on. You can produce a higher pressure by decreasing the area a force acts on.

Fluid Pressure A fluid is - - a liquid or a gas - a substance that will flow easily - able to change shape -exert pressure on surfaces they touch

Molecules of Fluids Fluids are made up of molecules - (extremely tiny particles. 1 L of water contains about 33 trillion trillion molecules. 33 & 24 0’s !!!) - in constant motion in all directions - constantly collide with each other and surfaces they contact

Molecules of Fluids All of the force exerted by the individual molecules in a fluid add together to make up the pressure exerted by the fluid.

Fluid Pressure Around Us You are surrounded by a fluid that presses down on you all the time. The atmosphere. (You have the weight equivalent to a washing machine on you!) This is called air pressure or atmospheric pressure.

This diagram explains why we don’t feel air pressure.

Air exerts pressure because it has mass. Each cubic meter of air = 1 kg - the force of gravity on this air produces pressure form the weight of the air. The atmosphere is over 100 km high.

Air pressure at sea level is about 10.13 N/cm2 Your hand is about 100 cm2 so the total force on your hand is about 1000 N.

Balanced Pressures How can you support the weight of the atmosphere? Remember: Pressure in a fluid is exerted equally in all directions - so if there is 10.13 N/cm2 on the top of your hand there is also 10.13 N/cm2 on the bottom.

Balanced Pressures So why aren’t you crushed by the atmosphere? Balanced pressures - pressure inside of your body is equal to that outside. Some of your body has air spaces (lungs, sinuses, ears) and part is filled with liquids. (cells, blood)

Look at what happens to this can if we pump all of the air out of it.

Why did it crush like this?

Balanced Pressures When the air is removed from the inside of the can, there is no air pressure pushing on the inside. So the air pressure on the outside has nothing to balance its push (nothing on the inside is pushing out) and the can crushes inward.

Variations in Fluid Pressure Pressure & elevation - - air pressure decreases as elevation increases - air pressure at a given point results from the weight of air from above. At higher elevations, there is less air above & so less weight. This is why your ears pop in an elevator or airplane. If the pressure in your ears is higher than outside, your body equalizes the pressure and this is the pop you hear.

Pressure & Depth Fluid pressure depends on depth. Water pressure increases with depth due to the weight of the water from above.

Pressure of water - also remember the weight of the air is pushing on top of the water. The deepest parts of the ocean may have 1000 times the pressure of the air.

You can feel the higher water pressure at the bottom of a pool.

Section 11-2 Transmitting Pressure in a Fluid Pascal’s Principle - when force is applied to confined fluid, an increase of pressure is transmitted equally to all parts of the fluid.

Pascal’s Principle - note the magnitude of the force when the stopper is pushed further in.

Force Pumps A force pump causes a fluid to move from one place to another by increasing the pressure in the fluid. Your heart consists of two force pumps. One pumps blood to the lungs & the other returns the blood to the heart.

Using Pascal’s Principle A force applied to one piston increases the pressure in the fluid. Note: the pistons are equal in area. So they experience the same force.

The left piston has the area of 1 square meter & the right piston has an area of 20 square meters. Then if you pushed down on the left piston with the force of 500 N, you increase in pressure on the fluid is 500 N/m2. Remember: pressure = force/area p = 500N/1m2

This means there is 500 N of force on every square meter. Since the surface area of the right piston is 20 square meters, what is the total increase of force?

20,000 N The left piston’s force has been increased or multiplied by 20 times! Depending on the size of the pistons, you can multiple force by any amount.

Hydraulic Systems Hydraulic systems are designed to take advantage of Pascal’s Principle. A hydraulic system multiplies a force by applying the force to a small surface area. The increase of pressure is then transmitted to another part of a confined fluid, which pushes on a larger surface area.

The brakes in your car is a common hydraulic system. This allows a person to push on a small pedal and stop a large vehicle.

Sea stars use hydraulics in their tube feet to move around and to cling to rocks.

Section 11-3 Floating and Sinking Why does something that floats also …

sink?

Buoyancy Buoyant force - a force that water exerts. The buoyant force acts in the upward direction, against the force if gravity, so it makes an object feel lighter. Objects feel less heavy in water.

Buoyant Force

Pressure is exerted on all surfaces of a submerged object Pressure is exerted on all surfaces of a submerged object. Since the pressure in a fluid increases with depth, the upward pressure is greater at bottom of the object than the pressure at the top. The result is a net force in the upward direction. This is BUOYANT FORCE.

Archimedes’ Principle States that: the buoyant force on an object is equal to the weight of the fluid displaced by the object. Archimedes was an ancient Greek mathematician.

This is a demonstration of displacement. Explain what you see.

A. B. C. Compare the direction of the buoyant force to the direction of gravity. Compare the size of the arrows. (Magnitude of force.)

B. Weight is greater than the buoyant force. A. B. C. A. Forces are balanced. B. Weight is greater than the buoyant force. C. The amount of water displaced is = to the weight of the object so now the forces are balanced.

Density Density = Mass / Volume

In terms of density, explain what A, B, and C did.

Will a rubber washer sink or float in corn oil?

This person is floating in the Great Salt Lake This person is floating in the Great Salt Lake. Why is he floating so high in the water?

Buoyancy and density.

The solid steel cube sinks (note its small buoyant force.) The same amount of steel spread into different shapes has different buoyant force & water displacement.

How does a submarine work??

How do these balloons float & sink?

Applying Bernoulli’s Principle states that: that the pressure exerted by a moving stream of fluid is less than the pressure of the surrounding fluid.

Try this: Take a piece of paper. Place 3 cm of a long edge between the pages of a text book. The paper should mostly hang free. Hold the book vertically so the paper hangs away from you. Blow air over the top of the paper. WHAT HAPPENS??

This is Bernoulli’s Principle in action. Blowing over the paper makes a moving stream & the pressure on top will be lower. The pressure under the paper will be higher so the paper will rise.

Chimneys work using Bernoulli’s Principle. How?

Air blowing over the top of the chimney causes the air pressure to be lower at the top of the chimney. This will make the smoke rise up and out of the chimney instead of filling the room.

Wings & Bernoulli’s

As air travels over the wing, it is forced to move farther & therefore moves faster. So the air moving over the top has less pressure. The higher pressure air underneath pushes UP . This is called LIFT.

Spoilers

Why does the shower move into the shower? Bernoulli’s can explain this. What is the moving fluid in the shower?

What is suction? Suction results from unequal air pressure. Examples: using a straw, a vacuum cleaner, medicine droppers, suction cups, your lungs.

The atomizer.

Bernoulli’s at work. A squeeze of the bulb causes air to move quickly over the top of the tube which extends down into the liquid.The greater pressure in the flask causes the liquid to spray out.

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