Physics 124A-Archimedes Principle
SAN DIEGO MESA COLLEGE
PHYSICS 124A LAB REPORT
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TITLE:
Archimedes’ Principle
(Buoyancy and Specific Gravity)
Objective:
To verify Archimedes’ principle and use it to determine the specific gravity of solids and liquids.
Theory:
Archimedes’ principle states that the buoyancy force B on an object immersed in a liquid is
equal to the weight W of the liquid displaced:
B  W
The weight W of the liquid displaced can be found by collecting the liquid and weighing.
If the object sinks in the liquid, the buoyancy force can be found from the difference between its
weight in air and in the liquid:
B  Win air  Win liq
The specific gravity of a substance is defined by
S .G. 
density of subs tan ce
density of water
Archimedes’ principle can be used to determine the specific gravity of various substances.
Case 1: A solid that sinks
The weight of the solid is given in terms of its volume V and density  S by
WS   SVg
If the solid sinks in water, it is completely immersed, the buoyancy force according to
Archimedes’ principle is
B  WVg
where W is the density of water.
From these two equations, it follows that
Physics 124A-Archimedes Principle
S .G. 
 S WS

W
B
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(1)
which determines the specific gravity in terms of WS and B .
Case 2 A floating object
A floating object can be made to sink by tying a piece of metal (the sinker) underneath it. The
apparent weight of the assembly when completely immersed in water is less than the apparent
weight when only the sinker is immersed. The difference is the buoyancy force on the object.
This can be used in Eq (1) to find the specific gravity of the object.
Case 3 Liquid
The buoyancy force on a piece of metal depends on the liquid in which it is immersed, and is
given according to Archimedes’ principle by
B  Vg
where   is the density of the liquid and V is the volume of the metal. It can be found by
weighing the metal in air and in the liquid. If the liquid is water, the buoyancy force is
BW  WVg
From these equations, it follows that the specific gravity of the liquid is
S .G. 
B
BW
Equipment: Overflow Can
Catch Bucket
Wood Cylinder
Lead cube
Aluminum Cylinder
Hooked Masses
Thin String
Double Pan Balance
Balance Elevation Mount
Scissors
1000mL Ungraduated Cylinder
Detergent
DeIonized Water
Electronic Balance
Physics 124A-Archimedes Principle
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In this experiment, all weights will be recorded in gram weight, or simply gram.
PART I: Verification of Archimedes’ principle and specific gravity of sinking solids
Setup:
Procedure:
Use the electronic balance to find the weight of the lead cube in grams. Make sure the balance reads
00.00 before weighing. If it doesn’t then momentarily push the “tare” button, wait a few seconds
and it should read zero. Remember weight is a measure of force.
Measure the weight of the catch bucket.
In this part of the experiment you will be suspending the samples from the hooks underneath the pans of
the double pan balance. Do this using a piece of thread that is long enough to get the mass
submerged in the bucket and still remain above the table.
Hang the lead weight from the bottom of the pan on the left.
Hold it out of the way and move the overflow can directly below the hook.
Have your lab partner fill the overflow can with deionized water (NOT tap water), add a drop of
detergent and all the excess to run out onto a paper towel.
Carefully lower the lead cube into the water, completely submerge it and catch what spills out through
the spout with the catch bucket.
Weigh the catch bucket and the water together.
Now suspend the hooked masses from underneath the other pan until the two pans are balanced (they
have equal masses hanging from them). The weight of these hooked masses is equal to the
weight of the submerged sample.
Finish filling in the data table for lead, then repeat the procedure for the aluminum sample.
Physics 124A-Archimedes Principle
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Data and Analysis:
Weight of catch bucket Wempty =
Lead
1. Weight of sample in air WS  Win air
2. Weight of sample in water Win water
3. Weight of catch bucket
and water W fill
4. Buoyancy force B  Win air  Win water
5. Weight of displaced water W  W fill  Wempty
% difference between 4 and 5
Specific gravity WS B
PART II: Specific Density of Floating Solids (Do Not use data from PART I)
Setup:
Aluminum
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Procedure:
In this experiment you will use a cylinder instead of the can to hold water. Half fill a cylinder with
deionized water.
Weigh the wood sample.
Hang the lead or aluminum weight from the bottom of the wood sample.
Weigh the assembly with only the metal immersed in water using the double pan balance by following
the procedure described in Part I.
Weigh the assembly with both wood sample and metal immersed. You might have to add more water to
the cylinder.
Data and Analysis:
Weight of wood sample WS 
Weight with sinker submerged W1 
Weight with sinker and sample submerged W2 
Buoyancy force on sample B  W1  W2 
S.G. of sample  WS B
PART III: Specific Gravity of Liquids
Procedure:
The specific gravity of two liquids, ethylene glycol and propanol, will be determined in this part
of the experiment. This is achieved by weighing the aluminum sample in the two liquids, one
after the other. The liquids will be provided to you contained in plastic cylinders.
The weights of the sample in air and in water are also needed for analysis, but they have already
been found in Part I.
WARNING: ETHYLENE GLYCOL and PROPANOL CAN BE HAZARDOUS TO YOUR
HEALTH IF MISHANDLED.
Physics 124A-Archimedes Principle
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Data and Analysis:
Weight of aluminum in air Wair =
Weight of aluminum in water Wwater =
Buoyancy in water BW  Wair  Wwater
Ethylene Glycol
Weight of aluminum in liquid W
Buoyancy in liquid B  Wair  W
Specific gravity  B BW
Conclusions and Comments:
Propanol