Date post: | 28-Dec-2015 |
Category: |
Documents |
Upload: | rudolf-wilson |
View: | 216 times |
Download: | 1 times |
Sect. 10-7: Buoyancy/Archimedes Principle• Experimental facts:
– Objects submerged (or partially submerged) in a fluid APPEAR to “weigh” less than in air.
– When placed in a fluid, many objects float!
– Both are examples of BUOYANCY.
• Buoyant Force: Occurs because the pressure in a fluid increases with depth!
P = ρg h (fluid at REST!!)
Archimedes Principle The total (upward) buoyant force FB on an object of volume V
completely or partially submerged in a fluid with density ρF:
FB = ρFVg (1)
ρFV mF Mass of fluid which would take up same volume as object, if object were not there. (Mass of fluid that used to be where object is!)
Upward buoyant force
FB = mFg (2)
FB = weight of fluid displaced by the object!
(1) or (2) Archimedes Principle
Proved for cylinder. Can show valid for any shape
Archimedes Principle
• Object, mass m in a fluid. Vertical forces are buoyant force, FB & weight, W = mg
• “Apparent weight”
= net downward force:
W´ ∑Fy = W - FB < W
Object appears
“lighter”!
Archimedes Principle & “Bath Legend”
• Archimedes Principle: Valid for floating objects
FB = mFg
= ρFVdispl g
(mF = mass of fluid
displaced, Vdispl =
volume displaced)
W = mOg = ρOVOg
(mO = mass of object,
VO = volume of object)
Equilibrium: ∑Fy = 0 = FB - W
• Archimedes Principle: Floating objects
Equilibrium: ∑Fy = 0 = FB -W
FB = W
or ρFVdispl g = ρOVOg
f = (Vdispl/V) = (ρO/ρF) (1)
f Fraction of volume of floating object which is submerged.
Note: If fluid is water, right side of (1) is specific gravity of object!
• Example: Floating log
(a) Fully submerged: FB > W
∑Fy = FB -W = ma (It moves up!)
(b) Floating: FB = W or ρFVg = ρOVg
∑Fy = FB -W = 0 (Equilibrium: It floats!)
Prob. 33: Floating Iceberg!(SG)ice= 0.917 (ρice/ρwater), (SG)sw= 1.025 (ρsw/ρwater)
What fraction fa of iceberg is ABOVE water’s surface? Iceberg volume VO
Volume submerged Vdispl
Volume visible V = VO - Vdispl
Archimedes: FB = ρswVdisplg
miceg = ρiceVOg
∑Fy= 0 = FB - miceg ρswVdispl = ρiceVO
(Vdispl/VO)= (ρice/ρsw) = [(SG)ice/(SG)sw] = 0.917/1.025 = 0.89
fa = (V/VO) = 1 - (Vdispl/VO) = 0.11 (11%!)
Example 10-9: Hyrdometer
(ρO/ρF)= (Vdispl/V)
Prob. 22: Moon Rock in water
Moon rock mass mr = 9.28 kg. Volume V is unknown. Weight W = mrg = 90.9 N
Put rock in water & find “apparent weight” W´ = mag “apparent mass” ma = 6.18 kg W´ = 60.56 N. Density of rock = ρ (mr/V) = ?
W´ ∑Fy = W - FB = mag . FB = Buoyant force on rock.Archimedes: FB = ρwaterVg. Combine (g cancels out!):mr - ρwaterV = ma . Algebra & use definition of ρ: V = (mr - ma)/ρwater. ρ = (mr/V) = 2.99 103 kg/m3
Example 10-10:Helium Balloon• Air is a fluid Buoyant force on
objects in it. Some float in air.
• What volume V of He is needed
to lift a load of m=180 kg? ∑Fy=0
FB = WHe + Wload
FB = (mHe + m)g , Note: mHe = ρHeV
Archimedes: FB = ρairVg
ρairVg = (ρHeV + m)g
V = m/(ρ air - ρ He)
Table: ρair = 1.29 kg/m3 , ρHe = 0.18 kg/m3
V = 160 m3
Prob. 25: (Variation on example 10-10)
mballoong
Fbuoy
mcargog
mHeg
Spherical He balloon. r = 7.35 m. V = (4πr3/3) = 1663 m3
mballoon = 930 kg. What cargo mass mcargo can balloon lift? ∑Fy= 0 0 = Fbouy - mHeg - mballoon g - mcargogArchimedes: Fbouy = ρ airVgAlso: mHe = ρHeV, ρ air = 1.29 kg/m3, ρHe = 0.179 kg/m3
0 = ρairV - ρHeV - mballoon - mcargo
mcargo = 918 kg