K-12 Teaching and Learning From the UNC School of Education

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Density is essentially how much stuff (mass) is within a defined space (volume). The more stuff in a specified space, the more dense it is. For example, a ping-pong ball and a golf ball are about the same size. But a ping-pong ball has much less stuff inside it and so, compared to the golf ball, it is less dense.

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A key concept in engineering concerns the properties of materials. What something is made of, how it is made, and how it interacts with other materials and substance determines how the material can be used.

By second grade, most children have experience with floating and sinking. In this lesson, students will learn to make things flink, meaning they neither float to the top nor sink to the bottom of a fluid. They will discover that whether an object floats or sinks depends not only on the properties of the object itself, but also on the properties of the fluid (either gas or liquid) in which it is situated. The pull of gravity on an object must counteract the push of buoyancy on an object in order for the object to sink. Similarly, the “push” of buoyancy on an object must overcome the pull of gravity in order for an object to float. Equilibrium is achieved when the two forces are even.

NOTE: These activities require a triple beam balance to use for measuring weight. One per group of students is ideal but in many elementary schools, few are available. Borrowing several from a nearby middle or high school is a possibility. Another possibility is to use a digital gram scale, or to do the activities as centers, or use bucket balances instead.

Learning outcomes

Students will understand that:

  • All objects and substances, including fluids (gases and liquids) have properties that can effect how they interact with each other.
  • Whether an object floats or sinks depends not only on the properties of the object itself, but also on the properties of the fluid in which it is situated.

Students will know:

  • Gravity is the downward force that pulls objects towards the earth.
  • Buoyancy is the upward force of a fluid that pushes up on an object.
  • Equilibrium exists when objects or forces are in a state of balance.
  • Weight and density are related but not the same.

Students will be able to use a triple beam balance to measure weight.

Essential question

What properties or characteristics determine whether an object will float or sink?

Time required for the activities in this lesson

  • Floatless Boats: 45–60 minute session
  • Measuring Weight: 45–60 minute session
  • Balloon Flinkers: 45–60 minute session


Activity 1: Floatless boats


  1. At each workstation, place the following items:
    • variety of items that float or sink: styrofoam packing peanuts, magnetic counters (or other small plastic item), metal washers, paper clips, corks, pennies, pieces of sponge, bottle caps, etc.
    • small clear container of water (like a clear plastic shoe box)
    • small towels
    • pencils
    • scissors
    • Floatless Boats data sheet

    If needed, alter the Floatless Boats data sheet to reflect the items the students will be using. Provide one data sheet for each student.

  2. Start a word bank on the wall for the vocabulary related to these activities.


  1. With the whole class, discuss things that float and things that sink. Why do some things float and others sink? (Answers will likely include that things that are big or heavy will sink and things that are small and light will float.) Ask students how big, heavy ships can float and why does a tiny pebble sink? Steer students towards making a distinction between what something weighs and how dense it is.
  2. With a small, clear container of water, use a paper clip and a packing peanut to demonstrate floating and sinking. Then pose the following question: “What can I do to these materials so that I create a Floatless Boat, something that neither sinks to the bottom of the container, nor rises to the surface? It ‘flinks.”” Accept all answers.
  3. Pose the engineering challenge: “You are going to test a variety of objects to see which float and which sink. Using the results of your tests, you will then design and build a Floatless Boat that will stay in the middle of the fluid.”
  4. Send students to their workstations. Students must first test each type of item on their table and record whether it is denser than the water (it sinks) or is less dense than the water (it floats).
  5. When students have tested each item, have them combine the objects in some way. They will place the item in the water and observe the results. To be considered a “flinker,” the item cannot have any part of it at the surface of the water, or any part of it touching the bottom of the basin.
  6. Have students clean and dry their area and gather together.


  1. Discuss the results of testing with students. Encourage students to think hard about what makes the difference between things that float and things that sink. Model the use of the words dense, density, less dense and denser as they describe their results. Add these words to the word bank.
  2. Introduce the concepts of gravity (the force that is pulling things down towards the earth) and buoyancy (the force that is pushing things up to the top of the fluid, in this case, water).
  3. Discuss equilibrium, the state of balance between forces. Add the word to the word bank.
  4. Begin a discussion of weight. Tell students that during the next activity, they will learn how to measure the weight of objects using a scientific tool called a triple beam balance. Have students retain their data sheets for reference during the next activity.


Evaluate student understanding throughout the activity and post-activity discussion. Allow students time to answer the questions at the end of the Floatless Boats data sheet. Check the data sheets for understanding.

Activity 2: Measuring Weight


  1. At each workstation, place the following items:
    • triple beam balance
    • 4 different sizes of wooden blocks
    • 5 each of the small items from the previous activity
    • pencils
    • Measuring Weight data sheet
  2. If needed, alter the Measuring Weight data sheet to reflect the items the students will be using. Provide one sheet for each student.
  3. Students will bring their Floatless Boats data sheet from previous activity.


  1. Talk about the results from the previous lesson. Ask students what they wrote down about what they think is responsible for why some things float and others sink. Ask them whether they think the weight of an object is a possible reason.
  2. Instruct students on how to use a triple beam balance. Demonstrate using a mid weight object. Tell students to use care with the balances.
  3. Tell students to measure the weight of each of the objects on their table. The larger objects will be measured one at a time. The smaller objects will be measured 5 at a time, otherwise the weight is too small for a triple beam balance to measure.
  4. Send students to work stations. Check progress as they work.


Gather students together. Discuss results. If the groups got different results, discuss possible reasons for discrepancy. Connect the work done with the triple beam balances to the previous activity. As students any closer to figuring out why some things float and some things do not. For instance, the corks are much heavier than the paper clips but they float and the paper clips do not. What is going on?


Check student papers for accuracy.

Activity 3: Balloon Flinkers


  1. Place supplies of the small items used in previous activities (magnet counters, paper clips, pennies, packing peanuts, etc.) on central supply table. Label the table PAYLOADS.
  2. Get helium balloons the day of the activity. Many grocery stores will give teachers 10–15 balloons free of charge or they will charge a small fee. Another alternative is to buy a small tank of helium from Party City, WalMart, or Target.
  3. If making your own helium balloons, add a ribbon about 4 feet long to the balloon.
  4. Cut a cardboard egg carton into separate 12 cups. Add piece of string to make a small egg carton bucket. This “gondola” will carry the payloads for the Balloon Flinker.
  5. Attach a gondola to each of the helium balloons. Keep the balloons out of sight until the students are ready to go to work.
  6. For each group of students, provide a triple beam balance.
  7. For each pair of students, provide pencils and scissors.
  8. For each student, provide a Balloon Flinkers data sheet and To Flink or Not to Flink worksheet.
  9. Students will bring their Floatless Boats and Measuring Weight data sheets from previous activities.


  1. Go over the results from the previous activities again. Talk about what floated and what sank. Get a few of the items that children identified as floaters and ask them what will happen if you dropped the item in water. Then, hold the item out in the air and ask what will happen if you dropped it in the air. Drop it. Ask why it did not float. Define the word fluid for students: a substance that flows or moves and has no shape of its own. Liquids and gases are both fluids.
  2. Bring out a helium balloon. Ask students why the balloon is floating. Encourage the use of the words density, gravity, buoyancy, etc.
  3. State the engineering challenge: “Students will try to determine how much weight the balloon gondola can carry while it flinks in the air for 20 seconds. To be a successful balloon flinker, no part of the balloon or the gondola can touch either the ceiling or the floor during the 20 seconds.”
  4. Students will choose one type of payload at a time. They may use as much of the payload as they like but they cannot mix the payloads.
  5. Students will weigh the payload before adding it to their gondolas and record the data on the data sheet.
  6. After recording the weight, students will add the payload to the gondola and let the balloon go. They will record the results with an up or down arrow, indicating the direction the balloon headed when released.
  7. Allow plenty of time for experimentation while insisting that students measure and record results accurately.


Discuss results with students. What was different in making things flink in water versus making it flink in the air? How were the forces of buoyancy and gravity working on the balloon? How does weight come into this? Tell students they are going to use their new knowledge to create a brand new toy.


Review student data sheets for accuracy.

Performance Task Assessment

Have students create a new toy that flinks. (They can actually make it or just draw/write about it.) Then, have students draw or write an ad for the toy that shows how and why it works. Students will be assessed on the clarity of their explanations, the correct use of words from the word bank.

Supplemental information

For an interesting article relating to the difficulties children encounter with the concepts of floating, sinking and density, see this article about challenges in understanding density.

Key vocabulary

A condition in which all forces acting on an object are canceled by others, resulting in a stable, balanced, or unchanging system.
Force of attraction exerted by a celestial body, such as Earth, upon objects at or near its surface, tending to draw them toward the center of the body.
The upward force that a fluid exerts on an object less dense than itself.
The measure of how much pull gravity has on a certain mass.
The quantity of something per unit volume, unit area, or unit length.
A substance that tends to flow and to conform to the outline of its container. A liquid or a gas.
A made-up word for something that neither floats nor sinks in a fluid.

  • North Carolina Essential Standards
    • Science (2010)
      • Grade 2

        • 2.P.2 Understand properties of solids and liquids and the changes they undergo. 2.P.2.1 Give examples of matter that change from a solid to a liquid and from a liquid to a solid by heating and cooling. 2.P.2.2 Compare the amount (volume and weight) of water...
      • Grade 3

        • 3.P.2 Understand the structure and properties of matter before and after they undergo a change. 3.P.2.1 Recognize that air is a substance that surrounds us, takes up space and has mass. 3.P.2.2 Compare solids, liquids, and gases based on their basic properties....
      • Grade 4

        • 4.P.2 Understand the composition and properties of matter before and after they undergo a change or interaction. 4.P.2.1 Compare the physical properties of samples of matter: (strength, hardness, flexibility, ability to conduct heat, ability to conduct electricity,...

North Carolina curriculum alignment

Science (2005)

Grade 2

  • Goal 3: The learner will observe and conduct investigations to build an understanding of changes in properties.
    • Objective 3.01: Identify three states of matter:
      • Solid.
      • Liquid.
      • Gas.
    • Objective 3.04: Show that solids, liquids and gases can be characterized by their properties.