MICROPHYSCI
KIT SAMPLE EXPERIMENT:
MicroPhySci Kit Experiment #11
11. BUOYANCY
Not included in the kit: thread
GOALS
1. To become familiar with buoyancy.
2. To measure the buoyant force on several objects.
BACKGROUND
We have all seen a boat floating on water. The boat floats because
the water is pushing up on the boat. This upward force is a buoyant
force and in this case is equal but opposite to the weight of the
boat.
In Experiment 2 you learned that when an object is placed in water,
the water will be pushed aside or displaced. It takes a force to
push the water aside (displace the water). This force is equal to
the buoyant force. This illustrates Archimedes’ principle
that states:
The buoyant force of a fluid (a gas or a liquid such as water)
on an object is equal to the weight of the fluid displaced.
In Experiment 1 you learned that the weight of an object is the
force of gravity on the object. This will not change when an object
is placed in water. However, the buoyant force is upward and will
make the object appear to weigh less.
Materials and Equipment
Aluminum (Al) bar
Polyethylene (PE) rod
Steel (Iron) bolt
Thread
Small rubber bands
Spring scale
50 mL graduated cylinder
PROCEDURE
Note: In this experiment
we will measure force (newtons) rather than mass (grams).
See the explanation of the relationship between force and
mass in Experiment 1 and Experiment 10.
1. Tie a thread about 20 cm long to the aluminum (Al) bar.
Suspend the rod from the spring scale with the thread (see
Figure 1), observe the weight, and record this in Table 1.
2. Pour about 25 mL of water into the graduated cylinder.
Now carefully lower the aluminum bar (still suspended from
the scale by the thread) into the water and, without allowing
the rod to touch the bottom of the cylinder, observe and record
the reading on the scale (Figure 2). The difference between
the weight in air (Procedure 1) and the weight in water is
the buoyant force. Record this in Table 1.
3. Repeat Procedures 1 and 2 for the steel bolt. Record your
results.
4. Weigh the polyethylene (PE) rod using Procedure 1. Remove
the thread.
5. You need to find the buoyant force on the PE rod without
using the scale. Pour exactly 25 mL of water into the graduated
cylinder. Slide the PE rod into the cylinder, being careful
not to spill any water. Without pushing the rod under the
water, note the level of the water and, by subtraction, find
the volume of water displaced when the rod is floating in
the water. Record this in Table 2.
6. Find the weight (in newtons) of one mL of water. Since
the density of water is 1.0 g/mL and there are 1000 g in one
kg, there will be 0.001 kg of water in 1 mL. We know that
one kg weighs 9.8 newtons (Experiment 1). If these are combined
we find that the weight of 1.0 mL of water is (0.001 kg)(9.8
N/kg). Calculate this and record it on the Results page.
7. From the volume of water displaced (Procedure 5) and the
weight of one mL of water (Procedure 6), calculate the weight
of water displaced. This is the buoyant force on the PE rod.
Record this in Table 1.
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FIGURE 1
FIGURE 2
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RESULTS, Buoyancy
Table 1
| Object |
Weight in air, N |
Weight in water, N |
Buoyant force, N |
| Al bar |
|
|
|
| Steel bolt |
|
|
|
| PE rod |
|
XXX |
|
1. Calculate and record the buoyant force on the aluminum rod and
the steel bolt..
2. Displaced volume of the floating PE rod =
Note: See Procedure 5.
3. Weight of one mL of water (Procedure 6) =
4. Buoyant force on the PE rod =
5. How does the weight of the PE rod compare with the buoyant force
on the rod? Why is this?
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