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 / float
Fb< 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 water
water / 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:

  1. Weight of the object.
  1. weight of the object in water
  1. apparent weight loss of the object

.98N-.686N=0.294N

  1. buoyant force of the object

0.294N

  1. density of an object
  1. 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.

  1. What is the density of the oak? What is the buoyant force on the oak block? What is the weight of the oak block?

  1. What is the density of the cork? What is the buoyant force on the cork block? What is the weight of the cork block?

  1. What is the density of object? What is the buoyant force on the object block? What is the weight of the block?

  1. Mrs. Yoon is 100kg and his volume is 0.09m3. Would he float in water? Explain why?

 Mr. U sinks

Another explanation

  1. 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

  1. 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?