Chapter 3: Forces in Fluids

Section 1: Fluid and Pressure Fluid: any material that can flow and takes the shape of its container Examples: liquid, gases Particles in fluids are able to slip by each other—allows them to take on container shape

All Fluids Exert Pressure Pressure: amount of force exerted on a given area Pressure = Force/area Pascal: SI unit for pressure

Why Are Bubbles Round? Fluid exerts pressure evenly in all directions to form sphere

Atmospheric Pressure Atmosphere: layers of Nitrogen, Oxygen, and other gases that surround the earth Extends to 150 km above surface 80% of gases found within 10km of surface Held in place by gravity Atmospheric Pressure: pressure caused by the weight of the atmosphere

Pressure Depends on Depth Pressure increases as you descend (deeper) More pressure at lower levels Your body will adjust to movement between higher and lower altitudes will cause your ears to ‘pop’

Water Pressure Increases with depth Depth effects pressure not the size of the body of water

Density Makes a Difference Water is 1000x more dense than air Density: amount of matter/volume

Section 2:Transmitting Pressure in a Fluid

Pascal’s Principle Blaise Pascal: 17 Century French scientist Pascal’s Principle: Change in pressure at any point in an enclosed fluid will be transmitted equally to all parts of a fluid

Force Pumps A pump causes a fluid to move by increasing pressure on the fluid Example: eye dropper, squeeze bottle, heart

Putting Pascal’s Principle to Work Hydraulic Lift: change sizes of piston to multiply forces Hydraulic: liquid pressure devise Multiplies a force by applying the force to a small area—increased pressure is transmitted through fluid—pushes on a larger surface Pushing on the petal transmits pressure to brake pads Starfish

Fluids go from High Pressure to Low Pressure Examples: liquid through a straw Example: breathing

Section 3: Floating and Sinking

Buoyant Force is Caused by Differences in Fluid Pressure Side pressures have net force of zero Because pressure is greater at bottom then at top—there is an upward net force

Determining Buoyant Forces Archimedes—Greek mathematician; 3rd Cent. B.C. Archimedes’s Principle: Buoyant Force is equal to the upward force; its equal to the weight of the volume of fluid an object displaces Weight of object not a factor

Weight Vs. Buoyant Force Sink Weight of object is greater than the weight of the fluid displaced Float Weight is equal or less than the weight of the fluid displaced Buoying Up If the force is greater than the object’s weight—then the object is pushed up until the weight of the displaced fluid equals the weight of the object

An Object Will Float or Sink Based on its Density More Dense Than Air Most substances more dense than air and therefore do not float Ex. Rubber Duckie less dense than water and so it floats in water. The Rubber Duckie is more dense than air and so it does not float in the air. Less Dense Than Air Helium is a gas that is less dense than air Ex. Balloons filled with helium float

The Mystery of Floating Steel Steel is 8x more dense than water and yet steel ships float Shape The shape of ships increases it volume Increase in volume decreases the ships density The ships overall density is then less than that of the water

Density on the Move Submarines dive because they take on and expel water How is a Fish Like a Submarine? The swim bladder can increase/decrease its volume—which increase/decreases the fish’s density

Section 4: Bernoulli’s Principle Daniel Bernoulli: 18th Century Swiss Mathematician Discovered that increasing the speed of a fluid decreases pressure; faster moving water exerts less pressure

It’s a Bird It’s a Plane It’s Bernoulli’s Principle Science in the Sink Tennis Balls are kept in the stream of water because the stream has less pressure than the surrounding air It’s a Bird It’s a Plane It’s Bernoulli’s Principle Faster moving air above wings exerts less pressure than slower moving air below the wings Increased pressure below causes and upward force for the wings/lift

Thrust and Wing Size Determine Lift Size of the wing and the speed at which air travels around the wing determines lift Trust—forward motion produced by engines

Bernoulli’s Principle Is For the Birds Birds Flap wings Larger wings need to be flapped less than smaller wings for birds to move

Drag Opposing Motion in Fluids Drag—force that opposes or resists motion in a fluid Turbulence—irregular flow of fluids Lift is often reduced when turbulence causes drag

Wings are Not Always Required Race cars and Frisbees also demonstrate this principle