This activity was adapted from the J. Chem. Ed. article, “Discrepant Event: The Great Bowling Ball Float-Off.” In this activity students use a bowling ball and some basic materials to predict whether the ball will sink or float in a tub of liquid.
Students in groups of 3 or 4 are assigned a bowling ball. For conventional bowling balls (circumference ranging from 26.704 inches to 27.002 inches) those weighing less than 12 pounds will float and those weighing more than 12 pounds will sink. I have a variety of bowling balls (the bowling alleys I visited were more than happy to donate a ball to science), so the answer to the question, “Will it float?” depends on which ball they were assigned. They may only answer the question using available materials: bowling ball, string, ruler, graduated cylinder, calculator, textbook, balance, and a handout of geometric equations. They are also asked further questions that probe their understanding of density.
Students will be able to:
1. Calculate density from mass and volume.
2. Convert from standard U.S. units to metric units.
3. Use density to predict whether an object will sink or float in water.
One bowling ball for each group of 3-4 students, a balance that can measure in pounds, string, rulers, graduated cylinders, cork rings to hold bowling balls, large tub of water.
I don’t mention the word “density” in my introduction of the activity though they should have done the textbook reading on density prior to coming to class. I offer a graduated cylinder to account for the holes, but in four years of doing this activity only one student has inquired about using one to make the volume correction. The results still come out fine if the volume correction is ignored. My 12 pound bowling ball eventually sinks though the average density that students calculate for this ball is slightly less than the density of the water.
You may want to pose the following questions to students:
Do the holes make a difference in your calculations?
Can you assume that the liquid in the tub is water? Does it make a difference what the liquid is?
Will the density of the bowling ball change if you cut it in half? Will it still float/sink?
A common misconception is that whether it floats depends on mass alone. I demonstrate this is not the case by putting a marble in the tub with a bowling ball that floats. Considering mass alone does not account for the fact that the marble sinks and the ball floats.
I collect the group activity sheets at the end of class and check if their procedure, calculations and conclusions are correct.
They have at least one or two exam questions on the chapter test that require them to apply density concepts and calculations. I haven't saved their responses in the past, but will do so when I use this activity in the future.
On the ACS General portion of the GOB standardized exam, they are required to calculate volume given the mass and density of a substance.
Usually when I check each group's original calculations during class, 3 to 4 groups out of nine will have made at least one calculation error (e.g. failure to convert from U.S. to metric units, incorrect calculation of radius). At the end of class usually 1 or 2 groups still have an error somewhere in their calculation. On occasion, one group may not make the correct relationship between their calculated density of the ball, the reported density of water, and whether the ball sinks or floats.
On the ACS standardized GOB exam, the number of students who were able to calculate volume given density and mass ranged from 67% to 75% over the past four years of teaching this introductory chemistry course.