13.3 Buoyancy
Buoyant Force (Fb)= fluid resistance
- Buoyancy = ability of a fluid (liquid, gas) to exert an upward force on an object placed in it.
- Buoyancy loss of weight of an object in a fluid
- Cause – pressure difference of the fluid between the top and bottom of the object - pressure at the top < pressure at the bottom of the object –
- Pressure at the bottom – pressure at the top = net force = buoyant force
- Determined by
1)medium of fluid: density of the fluid Fb
2)Volume of the object – for the same mass: volume Fb
Archimedes’ Principle
buoyant force = upward force = weight of a displaced fluid by the object.
Fb= Vfluid displaced× Dfluid× 9.8m/sec2 = ____ N
air
FbFb
Weight weight
water
Fpull = weight – FbFpull= force need to pull = apparent weight in water
Fb (in air) < Fb (in water) b/c Dwater>Dair will sink
Sink / Suspended / floatFb< weight / Fb= weight / Fb= weight
Dobject>Dfluid / Dobject=Dfluid / DobjectDfluid
Dobj of 0.8 g/cm3 in water: 80% in water, 20% on the water
Dobj of 0.5 g/cm3 in water: 50% in water, 50% on the water
Vobject=Vwater displaced / Vobject=Vwater displaced / Vobject>Vwater displaced
F = force
D = density =
V= volume (ml, cm3…)
Density (g/cm3) / Sink/float in waterwater / 1
ice / 0.92 / Float 92% in water
cork / .249 / Float 25% in water
gold / 20 / sink
Math problem- in lab
mass of the object in air (triple-beam balance) – 100g
mass of the object in water – 70g
volume of water displaced (by the whole object) –80ml
Calculate the following:
- Weight of the object.
- weight of the object in water
- apparent weight loss of the object
.98N-.686N=0.294N
- buoyant force of the object
0.294N
- density of an object
- Will the object float or sink in water?
Weight =0.98N
Buoyant force=.294N
Weight>buoyant force sink
another math problems.
In the water tank there are 4 objects of 100 cm3. The oak block is floating with 50% on the surface and 50% in the water. The cork block is floating 80% on the surface and 20% in the water. The block of object (unknown) is suspended in the water. The steel sank to the bottom of the tank.
- What is the density of the oak? What is the buoyant force on the oak block? What is the weight of the oak block?
- What is the density of the cork? What is the buoyant force on the cork block? What is the weight of the cork block?
- What is the density of object? What is the buoyant force on the object block? What is the weight of the block?
- Mrs. Yoon is 100kg and his volume is 0.09m3. Would he float in water? Explain why?
Mr. U sinks
Another explanation
- A 5cm3 block of lead (Pb) weighs .55N. The block was gently pushed and submerged in a tank of mercury (Hg). 1cm3 mercury weighs .13N
a)What is the weight of the mercury displaced by the submerged block?
Given: WPb=.55N, WHg=.13N/cm3
b)Will the block sink or float?
.55N<.65N float
- A steel block of 3cm × 4cm × 2cm was put in water. The density of steel is 7.8g/cm3. What is the weight of the block in water?
Given: Vsteel= 3cm × 4cm × 2cm =24cm3, Dsteel=7.8cm/cm3
The density of steel is 7.9g/cm. What is the buoyant force on the steel block? What is the weight of the steel block?