Review: Fluids. container into which it has been poured. changes gases are compressible. pressure changes

Transcription

1 Forces in Fluids

2 Review: Fluids o A fluid is a substance that is able to flow and assume the form of the container into which it has been poured o A compressible fluid is one that can change its volume when pressure changes gases are compressible o An incompressible fluid is one whose volume does not change when pressure changes

3 Pressure in Fluids o Pressure is the amount of force applied perpendicularly to an object per unit of surface area o Pressure is measured in Pascals (Pa): P = F A Where P = pressure (Pa) F = force (N) A = surface area (m 2 )

4 What factors affect the pressure in a fluid? Do you think the factors are different for gases and liquids?

5 Pressure in a Liquid o depends on two factors o Depth: o Increased depth = increased pressure o Density of the liquid o Higher liquid density = higher pressure

6 o Pressure depends on the number of collisions between gas particles o The more collisions, the greater the pressure o The number of collisions depends on: o Temperature o Increased temperature = increased pressure o Volume Pressure in a gas o Increased gas volume = decreased pressure o Number of particles o More gas particles = more pressure

7 Pascal s Principle o Pressure applied to one place of an incompressible fluid is transmitted unchanged throughout the fluid. o In some cases, a transfer of fluid can increase the force involved. o The pressure that results from a weak force on a small area can spread equally through a fluid in a closed system because the pressure remains the same throughout, the force that results on a larger surface area is larger

8 Pascal s example Force? Force = 10,000 N A B Area of A = 8 cm 2 Area of B = 50 cm 2

9 Break down the problem: 1 st solve for P in B P = Force Area 10,000 N P = 50 cm 2 A B P = 200 N cm 2

10 Break down the problem: 2 nd solve for F on A Force P = Area F = P x A A B F = 200 N cm 2 x 8 cm2 F = 1600 N

11 Archimedes Principle and Buoyant Force o What is buoyant force? o The upward force of a Fluid exerted on an object. o What is the downward Force exerted on an object called? o Gravity (the weight of the object) o Archimedes principle says that the buoyant force on an object in a fluid is equal to the weight of the fluid displaced by the object o In your lab on Archimedes principle, what did you find was the relationship between the buoyant force and the weight of the object and whether the object would float or sink?

12 Archimedes (Example #1) Mass = 20g Volume = 20ml Mass of Fluid= 20g

13 Archimedes (Example #1) Force of Gravity (F g ) = m of the object x g = (0.02 kg) x (9.8N/kg) = N The buoyant force = the force of gravity (Fb = Fg) object floats at the surface or just under the surface (dependent on its starting position) Buoyant Force (F b ) = m of fluid displaced x g = (0.02 kg) x (9.8N/kg) = N

14 Archimedes (Example #2) Mass = 50g Volume = 20ml Mass of Fluid= 20g

15 Archimedes (Example #2) Force of Gravity (F g ) = m of the object x g = (0.05 kg) x (9.8N/kg) = 0.49 N The buoyant force < the force of gravity (Fb < Fg) object sinks Buoyant Force (F b ) = m of fluid displaced x g = (0.02 kg) x (9.8N/kg) = N

16 Archimedes (Example #3) Mass = 30g Volume = 60ml Mass of Fluid= 60g

17 Archimedes (Example #3) Force of Gravity (F g ) = m of the object x g = (0.03 kg) x (9.8N/kg) = N The buoyant force > the force of gravity (Fb>Fg) object floats right on the top Buoyant Force (F b ) = m of fluid displaced x g = (0.06 kg) x (9.8N/kg) = N

18 How shape affects buoyancy

19 Archimedes Applications

20 Bernoulli s Principle o As the Velocity (speed) of a fluid increases, the Pressure exerted by the fluid decreases o When you have two areas of varying pressure, particles will move from areas of high pressure to low pressure. o How does this explain the experiments we did? o Experiment #1: Paper tent The sides of the tent will bend inward because the higher pressure air pushes toward lower pressure air

21 Experiment #2: Moving Balloons o The fast moving air between the balloons creates an area of low pressure o The normal/higher pressure on the outside pushes the balloons toward one another

22 Experiment #3: Balancing the Styrofoam

23 Experiment #4: the ball in the funnel o The air from your mouth creates an area of low pressure under the ball o The air above the ball is higher pressure pushing down on the ball, keeping it in place

24 Experiment #5: Magic Moving Ball

25 Experiment #6: cup of water

26 Bernoulli s principle and Liquids o Look at the diagram below, where would you expect pressure to be the lowest? o Flow Animation o What happens to the speed of the particles as we change the size of the pipe? What does this mean for the pressure based on Bernoulli s principle? o As the pipe narrows the speed of the particles increases o The faster the particles are moving, the lower the pressure this means that the pressure is lowest at point 2 in the diagram above

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28 Bernoulli s Principle and application to flight Creates Lift AIR The air on top of the wing must travel a longer distance than the air below the wing. The air on both sides must reach the end of the wing at the same time this means the air on top of the wing traveling faster = less air pressure. The difference in air pressure on top of the wing and under the wing creates lift

29 Lift Animation

30 LIFT DRAG THRUST GRAVITY