Real science for real life: Buoyancy


There are many different kinds of boats. Sometimes boat travel can be very dangerous, especially if the boats are being used for a different purpose than what they were designed for or if they are overloaded with too much weight.

8-25-16 SCIENCE Science for Real Life LogoThere are many scenarios – such as bathtub races, large cargo ships floating at sea, or even social issue such as refugees trying to escape on an overloaded boat – that might make students might wonder why some objects float while others sink? Science can help us understand why objects either sink or float, and we can use that knowledge and apply it through engineering to design and evaluate boats.

In order to help students understand this phenomenon, it is important that we understand the science behind buoyancy.

The phenomenon of buoyancy can be explained through understanding density and water displacement, which is partially determined by the object’s shape. The list below was developed to provide teachers with basic scientific principles.

  • Buoyancy: The ability to float in a liquid or rise in a gas.
  • Density: Mass divided by volume, expressed as “a unit of mass per a unit of volume.” Density is one property of an object that determines if it floats or sinks. Objects with a density LESS THAN water will float in water. Objects with a density GREATER than water will sink in water.
    • The density of water is 1 gram per cubic centimeter.
    • Here is a table of the densities of some common materials.
  • Water displacement: This occurs when an object is immersed in water. The immersed object pushes water out of the way and the water level in a container will therefore rise.
    • Any object placed into water displaces water. If the weight of the object is LESS THAN the weight of this displaced quantity of fluid, the object floats. If the weight of the object is MORE THAN the weight of this displaced quantity of fluid, it sinks.
    • The best way to visualize this is by using a graduated cylinder. Fill the cylinder half-full of water and add an object that sinks. You can measure the amount of water displaced by using the markers on the cylinder.
    • Using a graduated cylinder, you also can also see that an object that floats displaces water too. The amount of water displaced is equivalent to the volume of the object that is below the water’s surface.
    • Resource: This “Bill Nye the Science Guy” episode delves deeper and illustrates water displacement.
  • Why aren’t all boat hulls the same?

The shape of the hull of a boat influences how much water it displaces, how stable it is, how fast it will go and other characteristics. Not all boats have the same shape and not all boats serve the same function. Below are some basic shapes of hulls that can be found on boats. Do these different shapes displace different amounts of water? Would our observations about that be useful in designing a boat for a particular purpose?

10-27-16 SCIENCE - Buoyancy Column

  • How is buoyancy related to density?

In general, objects less dense than water will float. However, objects that are made of materials that are denser than water also may float depending upon their shape and resulting water displacement. This can be illustrated with a ball of crumpled up aluminum foil vs. a sheet of aluminum foil.

  • How do we measure density of an object?
  1. First measure the mass of the object on a scale.
  2. Then measure the volume of that object. Volume is easy to measure if we remember that an object displaces water equivalent to its own volume. The volume of small objects can be measured by dropping them into a graduated cylinder and reading the volume before and after. If the object floats, you have to immerse it just below the water to measure its volume. Measure the volume of larger objects by immersing them in a container that is full of water, then measure the amount of water that overflows
  3. Divide the mass by the volume, and express the density as unit of mass per unit of volume – such as grams per cubic centimeter.
  • How can we describe buoyancy as the balance of forces acting on an object?

The force of an object – like a boat – is pressing down on the water and displacing it. The force of the water also is pushing up on the boat. If there is salt – or more mass – in the water, the water is denser and it pushes up with more force, so objects in salty water will be more buoyant. Alternatively, if there is more mass in the boat it will push down more, displacing more water, and the boat will be less buoyant.

  • What happens when there is a hole in the boat?

Water entering the boat increases the weight inside, and therefore the density. The effect is the same as adding too many people to the boat. The density of the boat eventually exceeds the density of the water and the boat sinks.

Grades K-2 

At this grade band, students should engage in experiences that promote an understanding that different materials have different properties. Observe how different materials that are the same size have different weights. Observe how some materials float in water and some sink.

By the end of grade 2, students need to know:

  • PS 2.A: Objects pull or push each other when they collide or are connected. Pushes and pulls can have different strengths and directions. Pushing or pulling on an object can change the speed or direction of its motion and can start or stop it. An object sliding on a surface or sitting on a slope experiences a pull due to friction on the object due to the surface that opposes the object’s motion.
  • B: When objects touch or collide, they push on one another and can change motion or shape.

Suggested activities:

Fill a container with water and play with different floating and sinking objects. How do different pushes and pulls affect a floating object like a rubber duck? Can we push or pull it across the surface of the water? Can we blow on it to make it change direction? Can we make the duck sink underneath the water? What does it take to immerse the duck? How does the amount of force affect the motion of the object?

Grades 3-5

At this grade band, students should be able to understand that scientific theories are based on evidence and tests that allow us to alter explanations based on new evidence. What ideas do students have about what objects float and what objects sink? Can you introduce a new object and see how their perspective changes?

By the end of grade 5, students need to know:

  • A: Each force acts on one particular object and has both a strength and a direction. An object at rest typically has multiple forces acting on it, but they add to give zero net force on the object. Forces that do not sum to zero can cause changes in the object’s speed or direction of motion. (Boundary: Qualitative and conceptual, but not quantitative addition of forces are used at this level.) The patterns of an object’s motion in various situations can be observed and measured. When past motion exhibits a regular pattern, future motion can be predicted from it. (Boundary: Technical terms, such as magnitude, velocity, momentum and vector quantity are not introduced at this level, but the concept that some quantities need both size and direction to be described is developed.)
  • B: Objects in contact exert forces on each other – friction, elastic pushes and pulls. Electric, magnetic and gravitational forces between a pair of objects do not require the objects be in contact. For example, magnets push or pull at a distance. The sizes of the forces in each situation depend on the properties of the objects and their distances apart and, for forces between two magnets, on their orientation relative to each other. The gravitational force of Earth acting on an object near Earth’s surface pulls that object toward the planet’s center.

Suggested activities:

Give groups of students buckets or tubs of water and assorted objects of varying densities and shapes, such as:

  • metal or plastic bottle caps
  • aluminum foil
  • lumps of clay
  • things made of wood
  • things made of metal
  • paper
  • plastic bottles

Students will make observations and write questions while exploring the principle of buoyancy. Ask the students questions and ask them to make predictions as they make observations: How long does an object take to sink? How much do different objects weigh? Can we measure water displacement (volume)? What patterns do we observe related to the shape of an object and its buoyancy? If a lump of clay sinks, what happens when you flatten out the clay? Can you alter a floating object to make it sink? Can you alter a sinking object to make it float?

By the end of grade 5, students also need to know:

  • 3-5 ETS-1A: Possible solutions to a problem are limited by available materials and resources (constraints). The success of a designed solution is determined by considering the desired features of a solution (criteria). Different proposals for solutions can be compared on the basis of how well each one meets the specified criteria for success or how well each takes the constraints into account.
  • 3-5 ETS-1B: Research on a problem should be carried out – for example, through internet searches, market research or field observations – before beginning to design a solution. An often productive way to generate ideas is for people to work together to brainstorm, test and refine possible solutions. Testing a solution involves investigating how well it performs under a range of likely conditions. Tests are often designed to identify failure points or difficulties, which suggest the elements of a design that need to be improved. At whatever stage, communicating with peers about proposed solutions is an important part of the design process and shared ideas can lead to improved designs. There are many types of models, ranging from simple physical models to computer models. They can be used to investigate how a design might work, communicate the design to others and compare different designs.
  • 3-5 ETS 1C: Different solutions need to be tested in order to determine which of them best solves the problem, given the criteria and the constraints.

Suggested activity:

Challenge students to design a boat to hold as many passengers – pennies or washers – as possible. Students should be able to make predictions (hypotheses) about boat materials and boat shapes based on their observations. Students can make the boats, test the boats (How many pennies does it take to sink the boat?) and refine their designs based on their observations How can they improve upon the boat using what they have learned about density and water displacement?

Grades 6-8

At this grade band, students should be able to use various methods to conduct a scientific investigation, and use measurements to evaluate proposed explanations

By the end of grade 8, students need to know:

  • A: All substances are made from some 100 different types of atoms, which combine with one another in various ways. Atoms form molecules that range in size from two to thousands of atoms. Pure substances are made from a single type of atom or molecule. Each pure substance has characteristic physical and chemical properties – for any bulk quantity under given conditions – that can be used to identify it. Gases and liquids are made of molecules or inert atoms that are moving about relative to each other. In a liquid, the molecules are constantly in contact with each other; in a gas, they are widely spaced except when they happen to collide. In a solid, atoms are closely spaced and vibrate in position but do not change relative locations. Solids may be formed from molecules, or they may be extended structures with repeating subunits (e.g., crystals). The changes of state that occur with variations in temperature or pressure can be described and predicted using these models of matter. (Boundary: Predictions here are qualitative, not quantitative.)

Suggested activities:

Students can observe different materials that we expect to float or sink in water based on their density. What happens if we pour oil into a flask of water? Water into alcohol? Other combinations? You may color the water with some food coloring to make it easier to observe the interactions

What happens if we add salt to water? How does that affect buoyancy of different objects? Can we measure the density of the salt water and measure the density of different materials to explain the phenomena and make predictions about buoyancy? What has changed about the properties of the water when salt is added? What about other liquids such as rubbing alcohol? Vegetable oil? What properties do they have that are different from water and why? What do the molecules of these different materials look like? How does the shape of the molecule influence the property of the liquid?

By the end of grade 8, students also need to know:

  • MS ETS 1-A: The more precisely a design task’s criteria and constraints can be defined, the more likely it is that the designed solution will be successful. Specification of constraints includes consideration of scientific principles and other relevant knowledge that are likely to limit possible solutions (e.g., familiarity with the local climate may rule out certain plants for the school garden).
  • MS-ETS- 1 B: A solution needs to be tested, and then modified on the basis of the test results, in order to improve it. There are systematic processes for evaluating solutions with respect to how well they meet the criteria and constraints of a problem. Sometimes parts of different solutions can be combined to create a solution that is better than any of its predecessors. In any case, it is important to be able to communicate and explain solutions to others. Models of all kinds are important for testing solutions and computers are a valuable tool for simulating systems. Simulations are useful for predicting what would happen if various parameters of the model were changed, as well as for making improvements to the model based on peer and leader (e.g., teacher) feedback.
  • MS-ETS 1C: There are systematic processes for evaluating solutions with respect to how well they meet the criteria and constraints of a problem. Comparing different designs could involve running them through the same kinds of tests and systematically recording the results to determine which design performs best. Although one design may not perform the best across all tests, identifying the characteristics of the design that performed the best in each test can provide useful information for the redesign process – that is, some of those characteristics may be incorporated into the new design. This iterative process of testing the most promising solutions and modifying what is proposed on the basis of the test results leads to greater refinement and ultimately to an optimal solution. Once such a suitable solution is determined, it is important to describe that solution, explain how it was developed and describe the features that make it successful.

Suggested activities:

Students can design and make boats, optimizing for different qualities. How is a boat that is designed for speed or stability different from a boat that is designed to hold a lot of passengers or cargo?

Students can measure dimensions of model boats, calculate mass and volume, and predict how much weight each one should be able to carry before sinking. (Recall that if the weight of an object is less than the weight of its displaced quantity of fluid, the object floats. If the weight of the object is more than the weight of its displaced quantity of fluid, it sinks.)


Ashley Richards Best is a fifth year doctoral student and National Science Foundation graduate research fellow at the University of Louisville and Amanda Fuller is executive director of the Kentucky Academy of Science.