Instructor’s Copy
Lab Worksheet: Ship Ahoy
Background:
Fluids exert a pressure in all directions. The force that pushes upward in a fluid is called buoyancy. The upward buoyant force of a fluid opposes the downward force of gravity on an object placed in the fluid. According to Archimedes’ principle, the buoyancy of an object equals the weight of the fluid the object displaces.
Weight and mass are not the same thing. It happens, however, that it is more convenient to test Archimedes’ principle in the laboratory using measurements of mass rather than measurements of weight. The buoyancy of an object can be determined in terms of the mass of water the object displaces. This method works because mass and weight are proportional anywhere on the Earth.
If a solid piece of metal is placed in water, the metal will displace a volume of water equal to the metal’s own volume. One milliliter of water has a mass of 1 g. So the volume of water displaced equals the mass of the water displaced. And the mass of water displaced equals the volume of the object. The buoyancy of the object in water, therefore, is the same value as the volume of the object.
An object placed in water appears to lose mass. This apparent loss in mass is equal to the mass of the displaced water, or the volume of the object. So a 100-g object that has a volume of
25 mL will have an apparent mass of 75 g (100-25) when placed in water.
In this investigation you will determine the mass of a metal object in air. Then you will suspend the object in water and determine the mass again. You will then compare the apparent loss in mass with the volume of the displaced water.
Purpose: to see how buoyancy relates to the apparent loss of mass of an object.
Materials: centigram balance
25-mL graduated cylinder metal object
100-mL beaker thread ring stand with support ring wire gauze
Procedure:
A. Tie the metal object to the thread. Suspend the object in air by tying the thread to the hook
above the balance pan of the centigram balance. Record the mass of the object to the nearest .1 g
in the Data Table.
B. Set up the ring stand and support ring covered with wire gauze next to the balance so that a
beaker may be placed under the metal object.
C. Half-fill the beaker with water. Set up the metal object,
thread, and beaker of water as shown in
Figure 1. Do not remove or touch the balance pan.
D. Make sure the metal object is completely submerged
in the water and is not touching the sides or
bottom of the beaker.
E. Record the mass of the object in water to the nearest 0.1 g.
F. Place about 10 mL of water in the graduated cylinder. Record the volume to the nearest 0.1 mL.

G. Carefully place the metal object in the graduated cylinder. Record the new volume of the water
to the nearest 0.l1 mL.
Observations: Note: figures in the Data Table and “Observations” and “Analysis and Conclusions” are possible answers. Students’ numbers will vary, but the procedure remains the same.
Object measured
Mass of object in air
Measurement
12.5 g
Mass of object in water 8.4 g
Volume of water before adding object
Volume of water after adding object
10.0 mL
14.0 mL
1. Determine the loss of mass (buoyancy) of the object (mass in air – mass in water).
12.5 g = 8.4 g = 4.1 g
2. Determine the volume of water displaced by the object.
14.0 mL = 10.0 mL = 4.0 mL
3. Determine the mass of water displaced. Hint: 1 g of water = 1 mL of water.
4.0 mL = 4.0 g
Analysis and Conclusions:
1. Is the buoyancy of the object about the same as the mass of water displaced?
Yes; 4.1 g

4.0 g
2. Why is the loss of mass of the object only an apparent loss?
An object can never gain or lose mass.
Critical Thinking and Application:
1. A dancer finds that it is easier to lift a leg while standing in a swimming pool than while standing
on the floor. Why? The buoyant force of the water supports some of the weight of the leg,
making it “lighter.” Thus it is easier to lift.
2. People undergoing physical therapy after an injury often find it helpful to perform exercises in
water. Why? Because of the buoyant force of the water, it takes less effort to support and
move one’s body weight. Thus the injured person would find it less stressful to exercise in
water.
3. Explain why it is important in this investigation that the metal object not be touching the bottom
or sides of the beaker.
Some of the metal’s mass would be supported by the beaker, rather
than just by the water.
4. Suppose two blocks of identical shape and size were placed in water. Block A is made of wood
and block B is made of iron. How would the buoyant force on these two objects compare? Why?
Assume both blocks sink below the water line. Both buoyant forces would be the same, since
the volume of water displaced would be the same in both cases.
5. Explain what happens when a swimmer floats. The buoyant force of the water supports

enough of the swimmer’s body weight to keep the swimmer from sinking.