What Makes Objects Float? (Larissa Karan)

: Mass and floating.... what makes objects float?

Principle(s) Investigated:


Key Concepts

  Scientists use precise measurements to make accurate explanations.

  Heavier cubes are more likely to sink than lighter cubes (of equal volume).


Key Concepts

 Two different-sized objects made of the same material sink or float the same way.

 Heavy things may float or sink depending on their mass/ volume.


Key Concepts

  Objects with a density less than the density of water (1 g/cm3) will float in water at some level.

•  Under stable conditions, the density of a substance is a standard property of that substance and does not change.

Standards : Density and Buoyancy

All objects experience a buoyant force when immersed in a fluid. As a basis for understanding this concept:

a) Students know density is mass per unit volume.

b) Students know how to calculate the density of substances (regular and

irregular solids) from measurements of mass and volume.

d) Students know how to predict whether an object will float or sink.

MaterialsFor each group of 4–6 students

2–3 sets of cubes (Note: Do Not refer to these as “density” cubes with students as their inquiry with the cubes leads to exploration of density)

4 large rubber bands

calipers and/or rulers

balances (double balance and/or

graduated or electronic scale)

large container of water in which

cubes can float

towel to clean up spills

Procedure In this lesson, students discover for themselves the relationship between volume and mass for the density of solid objects. By gathering evidence about floating and sinking objects before being introduced formally to the concept of density (or the formula), students experience concrete connections to mass and volume being factors in determining what floats before being asked to understand the abstract notion of density.

Lessons 2.1, 2.2, and 2.3 engage students in making observations about cubes that sink and float that are later used to explain density. Students gather information about the mass and ability to sink or float for a number of single- and multi- cube objects, and then that data is graphed to reveal patterns for predicting what will float. When they graph their results and perform a careful analysis with the teacher’s assistance, the data reveals a linear relationship between mass and volume. The slope of that line is then explained as density.

Students will use technology to share their answers and create a graph showing a slope where objects will float or sink in water, using the following web link below:

Student prior knowledge: Students have participated in a floating puzzles activity, making observations, and know the difference between qualitative and quantitative observations.  They have also have learned that it is hard to predict whether various objects will sink or float.  (the floating puzzles included coke vs. diet coke, and vials of different shapes with the same mass, and vials of similar shape but different mass).  

2.2 Questions & AnswersREAPS Questions

R = What is the definition of a quantitative observation? A quantitative observation is one that can be measured. In other words, you can assign a number (quantity or amount) to the observation.

E = What is one factor that seems to affect whether an object is likely to sink or float? What evidence do you have that supports your idea? Mass (or weight) is likely the only factor that students will be able to explain with evidence from the class data.

A = Explain if the following statement is quantitative or qualitative: The copper cube sunk quickly to the bottom. This is a qualitative statement about the speed with which the copper cube sank because the speed was not measured directly.

P = If you had another set of cubes that were just the same as those you have now except they were all two times bigger, would the same kinds of cubes float as what you observed in this lesson? Yes. At this point, students have not been introduced to density being a standard property of solids, but they may make the connection that the same materials would behave the same way.

S = How have your ideas about how to predict what will float changed?

2.2 Questions & Answers: REAPS Questions

R = What unit do we use to measure mass? grams.

E = What is one qualitative observation that you made while testing multi-cube combinations? Students may have observed that the heavier cubes in the cluster rotated downward as the object sunk (or floated); they may have observed the relative speed at which the cubes sunk, or they may have observations about the colors of the cubes.

A = Explain if knowing the mass of an object is enough to predict if a multi-cube combination will float.  Just knowing the mass of an object is insufficient for accurately predicting if an object will float.

P = Is it possible that a cube that sinks in water would float in another liquid? Explain your answer. Some students may recognize that a cube that sinks in water might float in another liquid if the other liquid has a greater mass to volume ratio than water.

S = Were more of your predictions correct or incorrect about which cube combinations would float? Explain why.

Applications to Everyday Life: These activities help explain how heavy ships can float while smaller or lighter objects sink.  This can be extended to boating safety and how ships are designed.  


Videos: Include links to videos posted on the web that relate to your activity. These can be videos you have made or ones others have made.