Lecture 26

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# Lecture 26 - PowerPoint PPT Presentation

Lecture 26. Goals:. U nderstand pressure in liquids Use Archimedes’ principle to understand buoyancy Understand the equation of continuity Use an ideal-fluid model to study fluid flow. Fluids. At ordinary temperatures, matter exists in one of three states

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Presentation Transcript
Lecture 26

Goals:

• Understand pressure in liquids
• Use Archimedes’ principle to understand buoyancy
• Understand the equation of continuity
• Use an ideal-fluid model to study fluid flow.
Fluids
• At ordinary temperatures, matter exists in one of three states
• Solid: Has a shape and forms a surface
• Liquid: Has no shape but forms a surface
• Gas : No shape and does not form a surface

gas

liquid

solid

Increasing temperature

Fluids
• Fluids: Liquids or Gases
• Shape determined by container
• Close contact at boundaries. No macroscopic gaps
• Forces exerted by fluids are always normal to the surface.
• Fluids can flow
• Fluids:
• They have mass
• They have weight
• They carry energy
• They have velocity when flow
• Newton’s Laws apply
Fluids
• An intrinsic parameter of a fluid
• Density

units :

kg/m3 = 10-3 g/cm3

r(water) = 1.000 x 103 kg/m3 = 1.000 g/cm3

r(ice) = 0.917 x 103 kg/m3 = 0.917 g/cm3

r(air) = 1.29 kg/m3 = 1.29 x 10-3 g/cm3

r(Hg) = 13.6 x103 kg/m3 = 13.6 g/cm3

r(W or Au) = 19.3 x103 kg/m3 = 19.3 g/cm3

A

Fluids
• Another parameter: Pressure
• A Fluid exerts force on the objects it contacts
• Molecules constantly bounces off the surface with F =Σ(Δmv/Δt) (momentum transfer / time)
• Any force exerted by a fluid is perpendicular to a surface of contact, and is proportional to the area of that surface.
• Force (avector) in a fluid can be expressed in terms of pressure (a scalar) as:

Do not confuse pressure with momentum

What is the SI unit of pressure?

• Pascal
• Atmosphere
• Bernoulli
• Young
• p.s.i.

Units :

1 N/m2 = 1 Pa (Pascal)

1 bar = 105 Pa

1 mbar = 102 Pa

1 torr = 133.3 Pa

1 atm = 1.013 x105 Pa

= 1013 mbar

= 760 Torr

= 14.7 lb/ in2 (=PSI)

A bed of nails
• Distributing the force over a large area ….
Ping pong ball bazooka
• What is the initial force on the ping pong ball?
• Assuming this force is constant, how fast is the ping pong ball travelling when it leaves the tube?

F

This is a crude model….the speed of sound in air is 330 m/s

P0

F1

y1

y2

P1

A

P2

mg

F2

Pressure vs. DepthIncompressible Fluids (liquids)
• When the pressure is small, relative to the bulk modulus of the fluid, we can treat the density as constant independent of pressure:

incompressible fluid

• For an incompressible fluid, the density is the same everywhere, but the pressure is NOT!
• P(y) = P0 - y g r
• Gauge pressure (subtract P0)
• PGauge = P(y) - P0

F2 = F1+ m g

= F1+ rVg

F2 /A = F1/A + rVg/A

P0

F1

y1

y2

P1

A

P2

mg

F2

Pressure vs. Depth in water

Every 10 meters the pressure increases by 1 atm

Pascal’s Principle
• For a uniform fluid in an open container pressure same at a given depthindependentof the container
• Fluid level is the same everywhere in a connected container, assuming no surface forces
Exercise
• Does PA=PB?

B

A

(A)r1 < r2

(B)r1 = r2

(C)r1>r2

Exercise Pressure
• What happens with two fluids??
• Consider a U tube containing liquids of density r1 and r2 as shown:

Compare the densities of the liquids:

dI

r2

r1

Pressure Measurements: Barometer
• Invented by Torricelli
• A long closed tube is filled with mercury and inverted in a dish of mercury
• The closed end is nearly a vacuum
• PA=PB
• Measures atmospheric pressure as

1 atm = 0.760 m (of Hg)

• Consider the system shown:
• A downward force F1 is applied to the piston of area A1.
• This force is transmitted through the liquid to create an upward force F2.
• Pascal’s Principle says that increased pressure from F1 (F1/A1) is transmitted throughout the liquid.

P2

P1

P1 = P2

F1 / A1 = F2 / A2

A2 / A1 = F2 / F1

W2?

W1

W1 > W2

W1 < W2

W1 = W2

Archimedes’ Principle: A Eureka Moment
• Suppose we weigh an object in air (1) and in water (2).

How do these weights compare?

• Buoyant force = Weight of the fluid displaced

y

F

mg

B

Sink or Float?
• The buoyant force is equalto the weight of the liquid that is displaced.
• If the buoyant force is larger than the weight of the object, it will float; otherwise it will sink.
• We can calculate how much of a floating object will be submerged in the liquid:
• Object is in equilibrium

piece of rock

on top of ice

Bar Trick

What happens to the water level when the ice melts?

Expt. 1

Expt. 2

B. It stays the same

C. It drops

A. It rises

Exercise

V1 = V2 = V3 = V4 = V5

m1 < m2 < m3 < m4 < m5

What is the final position of each block?

Exercise

V1 = V2 = V3 = V4 = V5

m1 < m2 < m3 < m4 < m5

What is the final position of each block?

But this

Not this

For Tuesday
• All of chapter 14