# Forces in Fluids Ch. 11.

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Forces in Fluids Ch. 11

11.1 Pressure Essential Questions
What is pressure and how do you calculate it? What does pressure depend on? How do fluids exert pressure? How does pressure vary with change in altitude?

I. Pressure Pressure: the amount of force on an area.
A. Calculating Pressure = dividing force by the area where the force is applied. Ex: Pressure = force area

1. the SI unit for pressure is the pascal
2. One pascal (Pa) is equal to 1 Newton/meter squared

B. A Fluid is: 1. Any material that can flow and take the shape of its container. 2. Its particles can easily move past each other. ex: water, soda, oil, oxygen, carbon dioxide, air

C. Fluids Exert pressure equally in all directions.
Example – you cannot fill only half of a balloon

varies with: different levels of the atmosphere
D. Air pressure is pressure caused by the weight of the atmosphere and collisions with its molecules. varies with: different levels of the atmosphere Example: ears popping

2. pressure of fluids increases as the atmosphere gets deeper because at lower levels there is more fluid above you being pulled by gravity so there is more air pressure near the surface of the Earth. (pg: )

E. Water Pressure: 1. Water is a fluid and it exerts pressure like the atmosphere does. 2. water pressure increases with depth 3. water’s density is 1000 times greater than air

11.2 Sink or Float essential questions
1. How do you measure or calculate density? 2. How do you determine if an object sinks or floats? 3. What determines the buoyant force? 4. What is Archimede’s principle?

4. Density is mass of a substance compared to its volume
Ex: 100 ml of water has a mass of 100g: its density is 1g/ml 5. Because water is more dense than air, water exerts greater pressure than air.

II. Density F. Density is: mass per unit volume
1. density is calculated: density = mass volume 2. water’s density is 1 g/mL 3. every substance has its own density

4. By comparing densities, you can predict whether an object will sink or float in a fluid
5. Objects that are more dense will sink and objects that are less dense will float 6. By changing an object’s density, you can make it float or sink at different levels

III. Buoyant Force Buoyant Force is: the upward force that all fluids exert on all matter. A. Upward buoyant force is caused by greater pressure on the bottom of an object than there is on the top of the object.

B. Buoyant force opposes the pull of gravity.
That is why you feel “lighter” and can lift more under water

C. The weight of a submerged object is a downward force
If the weight is greater than the buoyant force the object will sink. If the weight is less than the buoyant force, the object will float

Archimedes’ Principle says buoyant force on an object is an upward force equal to the weight of the volume of fluid that the object displaces Buoyant force = weight of the volume of displaced fluid

2. An object’s weight has nothing to do with the buoyant force
The buoyant force is equal to the weight of the water that is displaced

C. Weight vs. Buoyant force
1. In fluids, objects will sink if their weight is greater than the weight of the displaced fluid (buoyant force acting on it). 2. Buoyant force opposes downward pull of gravity

C. Weight vs. Buoyant Force
3.) If the buoyant force is greater than the weight of an object, the object will be buoyed up out of the fluid. The bottom part of the object that is in the fluid displaces an amount of fluid equal to the objects entire weight.

D. Mystery of Floating Steel
1.) Shape of a boat can change volume 2.)If volume increases and mass stays the same, then density will decrease. 3.) Fish and submarines use buoyant force by controlling their overall density.

11.3-11.4 Pascal & Bernoulli essential questions
What is Pascal’s principle? Give Example What is Bernoulli’s Principle? Give examples

IV. Pascal’s Principle 1. says that a force on a confined fluid changes the pressure equally through the fluid 2. Fluids flow from areas of high pressure to areas of low pressure

3. liquids are used in hydraulic devices because they cannot be compressed or squeezed into smaller spaces.

V. Bernoulli’s Principle:
1. Bernoulli’s principle says as the speed of a moving fluid increases, its pressure decreases. (Fluid pressure increases as speed decreases.)

2. If air moves faster above the object, the fluid pressure pushes the object upward. If air moves faster below the object, the fluid pressure pushes it downward

III. Bernoulli’s Principle:
3. Faster moving air above a plane’s wing reduces the pressure, so increased pressure below the planes’ wing causes lift (an upward force)

B. Thrust and Wing Size Determine Lift
1.) Lift is determined by A) size of the wing B) speed of the air around the wing 2) Thrust is forward force produced by plane’s engines.

B. Lift 3.) Planes with greater thrust move faster and faster speed means greater lift. 4.) In fluids, the force that opposes motion is drag. 5.) Drag is usually caused by irregular flow of fluids known as turbulence. 6.) When turbulence causes drag, lift is reduced. 7.) Flaps are used to change the shape and area of a wing to reduce drag and increase lift.