What Floats Your Boat?

Standards covered: 8.2.b, 8.2.d, 8.8.c

Background Information

Imagine you’re floating on an inflatable raft along a winding, lazy river at a water park. What keeps you from sinking to the bottom of the river? In this lab, you will investigate forces that fluids exert on objects as you:

  • Measure the weights of objects in air and in liquids
  • Determine the buoyant forces exerted by the liquids on the objects
  • Determine the relative densities of liquids

Scientific Question

How can you determine the buoyant force exerted on an object by a fluid?

Hypothesize

Before step 8 in the procedure, write a hypothesis to tell how the weight measurement of an object in air will compare with the weight measurement of the same object in liquid. Your hypothesis should take the form of an “If…., then….,because” statement.

Procedure

1. Tie the string to the object. The string should be long enough to make a loop on one end of the object.

2. Have a partner hold the spring scale vertically and keep it steady.

3. Using the loop of string, suspend the object from the spring scale.

4. In the table, record the weight indicated on the balance to the nearest GRAM in the column labeled “Weight in Air.”

Table 1. Weight Measurements
Weight in Air (g) / Weight in Water (g) / Weight in Alcohol (g)
Trial / Trial / Trial
1 / 1 / 1
2 / 2 / 2
3 / 3 / 3
4 / 4 / 4
Average: / Average: / Average:

5. Repeat step 3, three more times so that you and your partner each have two turns holding the scale.

6. Calculate and record the average weight of the object in air.

7. Fill a beaker about 2/3 full of water. Lay a towel on the countertop. Place the beaker on the towel.

WRITE YOUR HYPOTHESIS NOW

______

______

8. Have a partner hold the spring scale with the object attached as before, but this time he or she should lower the object, slowly, until it is just submerged in the water. Make sure that the object does not touch the sides or the bottom of the beaker.

9. In the table, record the weight indicated on the balance to the nearest GRAM in the column labeled “weight in water.” Repeat step 4 three more times so that you and your partner each have 2 turns holding the scale. Calculate the average measurement. Blot the object dry with a towel and then allow it to air dry for about 5 minutes.

10. Fill the second beaker with alcohol, and repeat the process for weighing the object in this liquid. Record your data in the table.

Observe and Analyze

1. Calculate Determine the buoyant force that the water exerts on the object (FH20). To do this, subtract the weight of the object in water from its weight in air. Why is the object’s weight in water considered to be an apparent weight, or a weight that seems more or less than the actual weight? Repeat the calculation to determine the force the alcoholexerts on the object (Falcohol)

2. Identify Variables List the constants, independent variable, and dependent variable in this experiment.

3. Analyze Why didn’t you use the masses of the object in air and in liquids to determine FH20andFalcoholdirectly?

4. Analyze How do the buoyant forces of water and oil compare? What does this result suggest about the relative densities of these liquids?

Conclude

1. Analyze Compare your results with your hypothesis. Do the results support your hypothesis? Explain why or why not.

2. Identify Limits Discuss possible limitations or sources of error in your experiment.

3. Apply Why is it easier to stay afloat while using an inflatable raft than while swimming without one?

4. Apply Steel is used to build some large ships. Given that the density of steel is greater than the density of water, how are steel ships designed to float?