Physics--Chapter 9: Fluid Mechanics
Title: Archimedes’ Principle
Date:
Partner:
Objectives
Objects often seem to weigh less when immersed in a fluid than they do in air.
This is because the downward pull of gravity is diluted by an upward force, called the
buoyant force, that is provided by the fluid. If the buoyant force is large enough, it can
overcome gravity completely and cause an object to float.
The Greek philosopher Archimedes was intrigued by the principle of buoyancy
and reasoned that it had something to do with the liquid displaced by the submerged
object. Archimedes’ principle states “An object immersed in a fluid experiences an
upward force that is equal in magnitude to the weight of the fluid that is displaced by the
object”.
In this activity, you will test the validity of this principle.
Materials
TI-83 Plus graphing calculator w/ DataMate, LabPro interface, AC adapter, force sensor
w/ adapter, immersible objects (a 100-g and 200-g masses), string, overflow can, 2 small
beakers, water, glycerin, balance, ring stand, 2-way clamp, short metal bar
Procedure
1. Plug in the LabPro with the AC adapter, plug the force sensor w/ adapter into CH 1
on the LabPro. On the calculator under the APPS menu, open DataMate. When the
arrow appears next to CH 1, hit “enter” in the corner of the calculator to be able to
choose which sensor you are using. We are using the dual-range force sensor set to
10 N in this experiment. The mode for collecting data is irrelevant; you will not need
to save or graph data. We will simply monitor the results with the live read-out that
will appear on the screen.
2. Determine the mass of a small, empty, clean, dry beaker and record this value in the
data table. Fill the overflow can with water until the water just starts to flow out its
spout. When the water has stopped flowing, place the clean, dry beaker under the
spout of the overflow can.
3. Tie one end of a string around the object to be immersed and tie the other end to the
force probe. Mount the force probe on a ring stand so that the probe and object are
hanging vertically. Steady the object, and record the force. This will be the object’s
actual weight, Fact.
4. Submerge the object slowly into the water by lowering the ring stand. Be sure that
the water flows from the overflow can into the beaker. Lower the object until it is
just under the water’s surface. Record the weight of the object while it is completely
submerged in the water. This will be the object’s apparent weight, Fapp.
5. To determine the mass of the displaced water, determine and record the mass of the
beaker together with the displaced water. Subtract the mass of the empty, clean, dry
Physics--Chapter 9: Fluid Mechanics
beaker recorded earlier. Record this as the mass of the displaced water in the data
table.
6. Repeat this experiment two more times. For trial 2, use a different immersible object.
For trial 3, use a different fluid (glycerin).
Data Summary
DATA:
Trial 1
Fluid used:
Immersible object
used:
Mass of empty,
clean, dry beaker
Actual weight of
object, Fact
Apparent weight of
object, Fapp
Mass of beaker +
overflow
Mass of displaced
fluid
CALCULATIONS:
Weight of displaced
fluid
Buoyant force, Fb
Percent difference
Trial 2
Trial 3
kg
kg
kg
N
N
N
N
N
N
kg
kg
kg
kg
kg
kg
N
N
N
N
%
N
%
N
%
Results and Calculations
1. For each trial, determine the weight of the displaced fluid using the formula F = m  g
and record this in the table above.
2. Determine the buoyant force, Fb, for each trial by subtracting the object’s apparent
weight in the fluid, Fapp, from its actual weight in air, Fact. Record these values.
3. According to Archimedes’ principle, the buoyant force acting on a submerged object
is equal to the weight of the fluid displaced. How well did these values agree?
Calculate a percent difference for each trial and record these above as well (percent
difference is found by dividing the difference between the two values by the average
of the two values, and then multiplying by 100).
4. Did the fluid used affect your results? Explain.