lecture 4 - umd physics · lecture 4 fluid statics and dynamics ... • water flows at 5.0 l/s...
TRANSCRIPT
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Lecture 4Fluid statics and dynamics
• Using pressure: Hydraulic Lift
• Archimedes principle (float or sink?)
• Continuity equation
• Bernoulli’s equation
• Applications
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Hydraulic Lift• Use pressurized liquids for work
(based on Pascal’s principle): increase pressure at one point by pushing piston...at another point, piston can push upward
• Force multiplication:
• Relating distances moved by pistons:
• Additional force to move heavy object thru’ :
V1 = A1d1 equal to V2 = A2d2
! d2 = d1A2/A1
!F = !g (A1 + A2) d2d2
A2A1
> 1p1 = F1
A1+ p0
equal to p2 = F2A2
+ p0 + !gh
! F2 = F1A2A1" !ghA2 # F1
A2A1
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Buoyancy: Archimedes’ principle• Buoyant force: upward force of a fluid (float)
• Buoyant force, = weight of displaced fluid,
FB!fVfg
fluid
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To float or sink?• Net force:
• Float or sink or static equilibrium for
• ...rather for 1st case pushed up till only partly submerged:
• 90% of ice underwater...
FB = !fVfg = w = !0V0g! Vf < V0
!avg. < !f or !avg. > !f or !avg. = !f
FB ! w
!fVfg
master formula
(Vf = Vo: fully submerged)
!avg.Vog object
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Example
• A 6.0 cm.-tall cylinder floats in water with its axis perpendicular to the surface. The length of the cylinder above water is 2.0 cm. What is the cylinder’s mass density?
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• steel plate sinks, but geometry (sides) allows it to dissipate more fluid than actual steel volume:
Boats
mo < !fgAh ! !avg. = moAh < !f
buoyant force
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Ideal fluid• incompressible (not so
good approximation for gases)
• laminar (steady) flow (not turbulent): velocity at given point is constant with time
• non-viscous (no resistance to flow a la no friction for solid object)
• irrotational (test paddle wheel won’t rotate)
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Equation of continuity (I)• Streamlines (path of
“particle of fluid”:e.g. colored drop of water in in stream)
• Flow tube: bundle of streamlines (“invisible pipe”)
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Equation of continuity (II)• Fluid not created/
destroyed/stored within flow tube
V1 = A1!x1 = A1v1!t(volume flowing across A1) = V2...
Q = v A (volume flow rate) constant
Flow faster in narrower part : e.g., water from tap
master formula
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Bernoulli’s equation• work and energy conservation applied to volume
of fluid in flow tube:
W1 = F1!x1 = p1V
W2 = !F2!x2 = !p2V
!U = mgy2 !mgy1 = !V g (y2 ! y1)
!K = 12mv2
2 ! 12mv2
1 =12!V
!v22 ! v2
1
"
master formulae
!K + !U = Wext.
by pressure of surrounding fluid
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Example• Water flows at 5.0 L/s through a horizontal pipe that
narrows smoothly from a 10 cm diameter to 5.0 cm. diameter. A pressure gauge in the narrow section reads 50 kPa. What is the reading of a pressure gauge in the wide section?
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Applications I: Venturi tube(Measuring speed of flowing) gas
• Combine master formulae: (i) continuity equation (ii) Bernoulli’s equation (equal y’s) and (iii) pressure vs. depth and (iv) pressure same along line
v1 = A2
!2!liq.gh
! (A21 !A2
2)
v2 = A1
!2!liq.gh
! (A21 !A2
2)
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Applications II: Airplane lift• Continuity and
Bernoulli’s equations