Determining
Buoyant Force
In this lab, you will analyze recorded data to determine the buoyant forces acting on objects.
Problem How does the buoyant force determine whether an object sinks?
Materials
string, rock, spring scale, can, plastic tub, sponge, paper towels, 100-g standard mass, wooden block tied to a fishing weight, 250-mL graduated cylinder
For the probeware version of this lab, see the Probeware Lab Manual, Lab 5.
Skills Measuring, Calculating
Procedure
Make a copy of the data table shown.
Tie one end of the string around the rock. Tie the other end to the spring scale. Suspend the rock from the spring scale and measure and record its weight in air in your data table.
Place the can in an upright position in the plastic tub. Completely fill the can with water. Wipe up any water that spills into the tub. CAUTION Wipe up any water that spills on the floor to avoid slips and falls.
Lower the rock into the water until it is completely submerged. Record in your data table the apparent weight in water of the submerged rock. Remove the rock from the can.
Without spilling any water, carefully remove the can from the tub. Pour the water from the tub into the graduated cylinder. Record the volume of displaced water in your data table.
Repeat Steps 2 through 5, first with the 100-g standard mass and then with the wooden block that is tied to a fishing weight.
To determine the buoyant force on each object, subtract its apparent weight in water from its weight in air. Record these values.
Calculate the weight of the water each object displaces. (Hint: 1.0 mL of water has a weight of 0.0098 N.) Record these weights.
Analyze and Conclude
Observing What force is responsible for the difference between the weight of each object in the air and its apparent weight in water?
Analyzing Data How is the buoyant force related to the weight of water displaced?
Forming Operational Definitions Define buoyant force and describe two ways you can measure it or calculate it.
Drawing Conclusions Explain what causes an object to sink or to float, using the terms buoyancy, weight, force, density, and gravity.