Download 1. Buoyancy Archimedes 2010

Purpose:
• To find the relationship between the
upward force on an object submersed in a
fluid (buoyant force), and the fluid itself in
terms of mass, weight, volume and
density.
Procedure
• Submerse metals into spill cup
• Catch the water in another cup of known
mass
• Record data (in a chart) on mass, weight,
volume and density of both object and
displaced water.
Data
wa
Brass
Al
Cu
Zn
Pb
ww
wa-ww
wdispl mmetal Vfluid ρmetal
Conclusion
• If all the metals weighed the same, then
why are the buoyant forces different?
• What relationship is there between the
metal objects and water each of them
displaced when submerged?
Buoyant Force (Fb)
• Archimedes’ Principle:
A body fully or partially
immersed in a fluid is
buoyed up by a net force
equal to the weight of the
fluid the body displaces.
Archimedes’ Experiment:
The archon wanted to know if his crown was pure gold,
or if it was filled with a more base metal
Buoyant Force
• If Fb > W, It rises (wood)
• If Fb < W, it sinks (stone)
Calculating Buoyant Force
• Since Fb=weight of fluid (mg)
displaced, you can calculate it
from the density and volume of
the fluid.
Fb   fluidV fluid g
• If it’s floating the
Vfluid = Vunder water
A candle floats so it is 60% sunk,
displacing 30 ml of water. Find the
buoyant force on the candle.
What’s the weight of the candle?
What’s the weight of the water it displaced?
A candle floats so it is 60% sunk,
displacing 30 ml of water. Find the
buoyant force on the candle.
Fb   fluidV fluid g
Fb = 1 gm/ml x 30 ml x 9.8 m/s2
Units don’t match !
Fb = 1 x 10-3 kg/ml x 30 ml x 10 m/s2
Fb = .3 kg m/s2 = .3 N
What’s the weight of the candle?
30 ml 30 gm.03 kg  .3N
What’s the weight of the water it displaced?
A pure MKS density unit for water is 1000 kg/ m3:
1 gm/ml x (1 kg/1000gm) x (1000 ml/1 liter ) x (1000 liter/ m3)
Calculate the buoyant force on the block.
Why is this not enough to support the block?
How much should the displaced water weigh?
If the density of water is 1 gm/ml, calculate the
volume of the block.
Specific gravity is an old fashioned way of
measuring the DENSITY of rocks and minerals in the
field by comparing their weight in air to weight in water
Since the density of water is equal to 1 gram/cm3,
the specific gravity (no unit) = density in gm/ml
For all cases:
For sinkers only :
SG = Weight of object in air / apparent weight lost in water
For floaters only :
SG = % sunk
Use water, a balance, a graduated cylinder and the
equations above to find the volume, SG, density and
weight of a rubber ball.
Rank these 3 objects by
density
As the candle burns down, how will
its floating level change?
Things that will change:
Things that will remain the same: