210 11-1
EXPERIMENT 11
ARCHIMEDES’S PRINCIPLE
I. THEORY
Archimedes’s Principle states that when an object is completely or partially submerged in
a fluid, either a liquid or a gas, the fluid exerts an upward force on the object equal to the
weight of the fluid displaced. This force is called the buoyant force.
The goal of this experiment is to accomplish two things:
• Verify Archimedes’s Principle by measuring the buoyant force on a metal object
with known density.
• Use Archimedes’s Principle to measure the density of a block of wood.
In this lab, we will compute the magnitude of forces from the readings of a triple beam
balance.
A balance is designed to measure the mass of an object. An “unknown” mass is placed
on one side of a fulcrum and known masses on the other side are moved until the system
balances. When the system is balanced, the net torque on the system must be zero. Each
torque is proportional to the acceleration due to gravity at the location of the balance.
Thus, the readings of the balance are independent of location (San Mateo, Mount Everest,
Mars, the Moon …) and depend only on the unknown mass.
A scale measures force, generally the tension or compression of a spring. A scale will
have different readings for the same object at different locations.
In this lab, we will measure actual masses and apparent masses with the triple beam
balance. In the standard use of a balance, it is assumed that the only forces acting on the
“unknown” mass are a gravitational force downward and an upward force from the
balance. At times, we will have three forces on the “unknown” mass, a downward
gravitational force, an upward force from the balance (the tension in a string), and an
upward buoyant force. The upward force from the balance will be equal in magnitude to
the balance reading times the acceleration due to gravity. The balance reading can be
referred to as the apparent mass of the “unknown”.
II. LABORATORY PROCEDURE
1. In this experiment each lab group will be working with a balance. As always, zero your
balance.
2. Examine your metal object and determine whether it is made of aluminum, brass, copper
or iron (steel). Record the material on your data sheet. Measure and record the mass of
your metal object.
3. Measure and record the mass of the wood block.
4. Temporarily remove the pan of the balance. Raise the shelf to its highest position.
Replace the pan. Check that your balance is still zeroed.
5. Fill a beaker about ¾ full of water and place it on the shelf.
6. Attach a string to the metal object and suspend it from one of the hooks where the pan
attaches to the balance. Adjust the amount of water in the beaker and/or the length of the
