Fluid Pressure Chapter 13 Section 1 Page 390

Fluid Pressure Chapter 13 Section 1 Pg

Have you wondered…?

Pressure The result of a force distributed over an area So… – The larger the area over which the force is distributed, the less pressure is exerted. – Ex: a pencil point has a much smaller area than the eraser, so it exerts much greater pressure than the eraser.

Calculating Pressure

Units Force- measured in newtons (N) Area- measured in square meters (m 2 ) Pressure- SI unit is newton per square meter (N/m 2 ) – Also called pascal (Pa) 1 N/m 2 = 1 Pa Named for French scientist Blaise Pascal Pressure is often in kPa – 1 kPa = 1,000 Pa

Practice Problem Calculate the pressure produced by a force of 800 N acting on an area of 2.0 m 2. P = F/A P = (800 N)/(2.0 m 2 ) P = 400 N/m 2 = 400 Pa = 0.4 kPa

Fluids Fluids also exert a pressure – Fluid is a material that can easily flow – Fluids take shape of container – Liquids and gases are fluids

Cause of Fluid Pressure Fluid particles collide with a surface, exerting a force on the surface All of the forces exerted combine to make up the pressure exerted by the fluid The number of particles in a fluid is large, so a fluid can be considered as a whole.

Fluid Pressure Determined by the type of fluid and its depth

Fluid Pressure Water pressure increases as depth increases. The pressure at any given depth is constant, and it is exerted equally in all directions.

Why do ears “pop”?

Air Pressure Earth’s atmosphere is made up of a mixture gases Atmospheric pressure = 101 kPa at sea level Air pressure decreases as the altitude increases

Review

1. Order from lowest to highest pressure. A.A<B<C B.C<B<A C.all are equal

2. Look at the markers. Order from lowest to highest pressure. A. Y<Z<X B. Y<X<Z C. Z<X<Y D. X<Z<Y E. two are equal X Y Z

3. What will happen to the pressure if more water is added? A.increase B.decrease C.stay the same

4. What will happen to the pressure if more water is added while the same amount is removed? A.increase B.decrease C.stay the same

5. What will happen to the pressure if the fluid were changed to honey? A.increase B.decrease C.stay the same

6. If the 250 kg mass was put on the water column, what will happen to the pressure? A.increase B.decrease C.stay the same

7. If the only change was to remove the air pressure, what will happen to the pressure? A.increase by kPa B.decrease by kPa C.stay the same D.Something else

8. If the only change was to go to a place where the gravity was doubled, what will happen to the pressure? A.Both pressures would double B.Only the air pressure would double C.The air pressure would double, and the water pressure would increase some D.Something else

9. How do the pressures at the two locations compare? A.X>Y B.Y>X C.They are the same X Y

Review 1.What is the relationship between the depth of water and the pressure it exerts? 2.How is pressure distributed at a given level in a fluid? 3.How does the pressure exerted by the atmosphere change as altitude increases? 4.Why don’t you feel the pressure exerted by the atmosphere?

Review 5.A 500 N student stands on one foot. A 750 N student stands on two feet. If both students wear the same size shoe, which exerts the greater pressure? 6.A circus performer on a pair of stilts exerts a pressure of 32 kPa on the ground. If the performer stands on one stilt, what pressure does the stilt exert on the ground?

Forces and Pressure in Fluids Chapter 13 Section 2 Pg

Transmitting Pressure in a Fluid Recall: A fluid exerts pressure equally in all directions at a given depth. The amount of pressure exerted by a fluid depends on the type of fluid and its depth.

Pascal’s Principle Note that the pressure increases equally throughout the water, not just at the point where you squeeze- discovered by Pascal in 1600s. According to Pascal’s principle, a change in pressure at any point in a fluid is transmitted equally and unchanged in all directions throughout the fluid.

Hydraulic Systems A hydraulic system is a device that uses pressurized fluid acting on pistons of different sizes to change a force. An increased output force is produced because a constant fluid pressure is exerted on the larger area of the output piston.

Bernoulli’s Principle Get out a piece of paper Daniel Bernoulli- Swiss scientist – Principle: as the speed of a fluid increases, the pressure within the fluid decreases As the air blows across the top of the paper, the pressure exerted by the air decreases. Because the air below the paper is nearly motionless, it exerts a greater pressure. The difference in pressure forces the paper upward.

Bernoulli’s Principle Wings and Lift – Bernoulli’s principle help explain the flying ability of birds and airplanes. – Air traveling over the top of an airplane wing moves faster than the air passing underneath, creating a low-pressure area above the wing. – The pressure difference between the top and the bottom of the wing creates an upward force known as a lift – Wings of birds produce a similar lift to airplanes, but birds can flap their wings to produce forward movement and some lift – Wings can also be used to create a downward force: race cars often have a spoiler (upside-down wing) to give better traction

Buoyancy Chapter 13 Section 3 Page

Buoyant Force Buoyancy is the ability of a fluid to exert an upward force on an object placed in it Buoyancy results in the apparent loss of weight of an object in a fluid. When an object is submerged in water, the water exerts an upward force on the object, making it easier to lift. – The upward force acts in the opposite direction of gravity and is referred to as buoyant force

Buoyant Force How it is produced… – Since water pressure increases with depth, the forces pushing up on the bottom of the object are greater than the forces from pressure pushing down on the top of the object (any non-vertical forces cancel one another out), resulting in a net upward force- the buoyant force

Archimedes’ Principle Archimedes- an ancient Greek mathematician Archimedes’ Principle- the buoyant force on an object is equal to the weight of the fluid displaced by the object – When an object is submerged, it pushes aside, or displaces, a volume of fluid equal to its own volume. – When an object floats on the surface, it does not displace its entire volume. However, it does displace a volume equal to the volume of the part of the object that is submerged

Density and Buoyancy Recall: density is the ratio of an object’s mass to its volume Density is often expressed in units of grams per cubic centimeter (g/cm 3 ) – Water has a density of 1 g/cm 3 – Steel has a density of 7.8 g/cm 3 If an object is less dense than the fluid it is in, it will float. If the object is more dense than the fluid it is in, it will sink.

Density and Buoyancy The force of gravity, equal to the object’s weight, acts downward on the object. The buoyant force, equal to the weight of the volume of displaced fluid, acts upward on the object. When the buoyant force is equal to the weight, an object floats or is suspended. When the buoyant force is less than the weight, the object sinks. (see pg. 401)

Density and Buoyancy Suspended – When an object has the same density as the fluid it is submerged in. – Buoyant force exactly equals the object’s weight – Neutral buoyancy Sinking – When an object’s weight is greater than the buoyant force acting on it – Negative buoyancy

Density and Buoyancy Floating – When the buoyant force is greater than the object’s weight – Positive buoyancy – Objects float more easily in dense fluids – Heavy ships can float because of the shape of the hull. It displaces a large volume of water, creating a large buoyant force – The denser the fluid is, the greater is the weight displaced, which results in a greater buoyant force

Density and Buoyancy

See page 402 in your book for how a submarine works.

Works Cited Frank, Wysession, & Yancopoulos. “Chapter 13 Forces in Fluids.” Physical Concepts in Action. Upper Saddle River: Pearson, Print.