Density and Buoyancy

Our unit on floating and sinking is coming to a close. Remember that in standards-based grading, your grade is based on your content knowledge. It is never too late to improve. To jog your memory, here are two recent activities we've done in class.

We built and sunk aluminum foil boats. In the first part, we built a small, medium, and large boat. Then we put pennies in them until they sunk. Using our triple beam balance and graduated cylinder, we found the mass and volume. Using the formula: density = mass/volume we were able to find the density.

In part two, our task was to design boats that would hold a minimum amount and then sink. For example, the first boat needed to hold 15 pennies and sink by the time it got to 20. By finding the mass of 15 pennies, we were able to predict the volume the boat would need to be. This time we built a boat using the formula: volume = length * width * height.

Today we created rainbow density columns. The goals was to create a liquid rainbow in a graduated cylinder using only food coloring, sugar, and water. The less dense liquids floated on top of the more dense ones. Here are some examples.

Why do things float or sink?

We have just completed an experiment investigating why things float or sink. Students filled film canisters with various weights. The students found the mass when the film canister sunk, floated, and was at neutral buoyancy. They used water displacement to find the volume of the film canister. Here's the data from 15 of the groups.


Averaging the data we got a mean mass for the sinking items as 50.6. For floating items it was 16.8. For neutral items the mass was 40.5. The average volume we found was 40.2

Students then created a rule based on the data. It appeared that the objects where their mass was greater than their volume, they would float. If the mass was less, it would sink. If it was the same, it would be neutral.

The relationship between mass and volume is called density. It is the key topic we will be studying for the next two weeks.