Buoyancy: Testing boats of various materials

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SWBAT determine variables that affect the force of buoyancy.

Big Idea

Students investigate what items will sink and float, and then conduct an experiment to determine which boat,made of various materials, will hold the most weight.

Rationale and Preparation

The Why Behind Teaching This: 

This unit covers standard 5-PS2-1: Support an argument that the gravitational force exerted by Earth on objects is directed down.  During the unit, students will investigate a variety of objects to see that the force of gravity is constant on Earth and pulls things down towards its center.  We will also be investigating a variety of ways to overcome gravity. 

Several of the lessons in this unit are engineering design projects requiring students to follow the steps of the engineering design process to construct a project.  These projects address standard 3-5-ETS1-1: Define a simple design problem reflecting a need or a want that includes specified criteria for success and constraints on materials, time, or cost. It also addresses engineering standard 3-5-ETS1-2: Generate and compare multiple possible solutions to a problem based on how well each is likely to meet the criteria and constraints of the problem. There are also several experiments in the unit which address standard 3-5-ETS1-3: Plan and carry out fair tests in which variables are controlled and failure points are considered to identify aspects of a model or prototype that can be improved. 

This specific lesson addresses the standard on gravity by comparing the force of gravity to the force of buoyancy in relationship to a variety of objects.  Students investigate characteristics of items that float to those that sink to determine the relationship between these items.  They then apply what they have learned through the investigation to an experiment where they test boats made of a variety of materials.  By conducting an experiment which requires them to identify and control variables, the lesson also covers standard 3-5-ETS1-3. 

Lesson Goal: 

 The goal of this lesson is for students to demonstrate an understanding of the experimentation process while identifying characteristics that affect buoyancy and gravity.  

Success Criteria:

Students will demonstrate success on this lesson goal by working through the experiment steps independently and collecting data.  They will also demonstrate mastery by explaining what characteristic of the boat helped buoyancy overcome the pull the gravity the longest.  

Preparing for Lesson:

Warm Up:

Guided Practice: 

  • Five boats of the same size, one made of plastic, wood, aluminum, styrofoam, and cardboard. 
  • trifold experiment foldable for each student
  • a copy of the Boat Experiment Steps for students who need the modification 
  • a copy of the Boat Experiment Procedure for all students 


  •  five stations set up around the room for each test. Each station will need a tub of water, a hand towel, and weights. 

Wrap Up: 

Students will be sharing their results from the experiment.  No additional materials are needed. 

Warm Up

15 minutes

Sink or Float Investigation

Earlier in the unit, during the types of forces lesson, students tested a ball of clay and the same amount of clay flattened out, to determine if they would sink or float.  Through this activity students discovered that the surface area of an object has an affect on the force of buoyancy.  There are other things that can affect buoyancy that I would like students to connect to this previous experience.  

In order to facilitate the connection, I begin today's lesson with a ball of clay and a tub of water.  I remind students of the activity that they have already completed by placing the ball of clay in the water and watching it sink to the bottom.  I ask what force pulled the ball down, they tell me gravity.  I ask them what we could do to the clay to make it float.  They tell me flatten it out like a boat.  I ask them why it floats this time and they tell me there are more particles of water under it applying a force against gravity.  I place a marble on the clay and it pushes it down just enough to fill it with water, causing it to sink.  I ask why it sank when the marble was added.  Students tell me that the water filled it, making it heavier, which caused it to sink.  So, we know that surface area affects buoyancy, and weight can also affect buoyancy. I pose the question, What else do you think might affect buoyancy besides surface area? I explain that we are going to do some investigating to help us answer that question.  

I provide each student with a copy of the sink or float investigation lab sheet for their notebooks.  As they glue the notebook page in, I gather the items needed for the investigation.  I set out a can of Diet Coke, a can of regular Coke, a mini Snickers candy bar, a mini 3 Musketeers candy bar, an unpeeled orange and an orange of equal size that has been peeled. 


I show the students the items and ask them to record their prediction in the column titled "prediction".  Do they think each item will sink or float?  As they are recording their predictions, I circulate to see what students are recording.  All students miss at least one, the majority of students believe that both sodas will sink which is not correct.  Below is an example of one student's predictions. 


I begin by placing the diet soda and regular soda in the tub of water together.  As you can see in the photo below, the regular soda sinks all the way, the diet soda floats up a little higher.   I have students record their observations on the lab sheet.  I ask for some ideas on why they think the Diet Coke floats and the regular Coke sinks.  Students believe it is because the diet has less sugar.


I then unwrap both candy bars and place them in the tub of water.  It isn't clear in the picture but the 3 Musketeers is floating on top, and the Snickers sinks all the way to the bottom.  Students record the results on the lab sheet and then share some ideas on why they believe one sank and the other floats.  Students who have tried both candy bars tell me that the Snickers has more ingredients in it like peanuts and carmel where as the 3 Musketeers only has fluff in it.    


Our final test is on a peeled orange and an unpeeled orange.  I place both in the tub of water together.  The unpeeled orange, although it is larger, floats, and the peeled orange sinks.  Students record the results on the lab sheet.  Students offer some suggestions on why the peeled orange sinks when the unpeeled orange floats.  Students make the connection to the air bubbles in the fluff and tell me that the peel of the orange has little air bubbles in it.  Another student disagrees with that and says that the peel keeps water out, when it is unpeeled, water fills in the holes and empty spaces and sinks.   


Giving the True Explanations 

Students were able to come up with some pretty good reasons why the items above would sink or float.  I want to make sure they know the real reasons so I share with them the information about why each happens (explanations for the sink or float investigation).  As I discuss each, I read off the sugar content from the cans of soda, cut open a 3 Musketeers and show them the air pockets, and provide each group with a piece of the rind from the orange to observe.  Having the opportunity to see the differences first hand will make it easier for students to understand.  

Guided Practice

15 minutes

Setting Up the Experiment 

I provide each student with a trifold experiment foldable.  I use this style foldable with every experiment so that students are practicing the set up for our end of the year science fair.  It is also helpful to have a routine procedure for experiments since we conduct so many.  It helps the planning process go quicker.  I also provide the ESE/ELL students with a copy of the Boat Experiment Steps that have been precut and placed into a Ziplock bag.  This modification helps these students focus on information being presented instead of focusing on copying down information.  I have them precut so that students still have to read the information and match it to what I am recording so I know they are hearing and reading the information.  It takes them less time to read short inserts, then it does to write the information.  

We begin with the question: Which material boat will float the longest when weight is added?  I show students the five boats being used.  I have made boats of equal size out of Styrofoam, plastic, wood, cardboard and aluminum.  I created 5 boats because I have 5 groups and they will be rotating around to test one boat at a time.  The cardboard boat has to be lined with duct tape because the cardboard starts to get soft when it gets wet.  I always keep a backup for the cardboard boat. 


Next, we record the hypothesis: If I place boats made of Styrofoam, wood, plastic, cardboard and aluminum in a tub of water and add weight to them, then I predict the ___________________ boat will continue floating with the most weight in it.  I circulate to allow students to hold and observe the differences between each boat.  Making these observations will help them complete their hypothesis.  Students are making the connection between the sink or float investigation at the beginning of the lesson because I hear things such as "the Styrofoam has air pockets in it so it will hold the most weight".  I allow students to discuss their observations and hypothesis with their peers at their table group but we do not discuss it whole group.  I don't want students to be influences by others.  After everyone has their hypothesis filled in, I mention some of the observations I heard being discussed and point out that the purpose of the experiment is to test and see if any of these ideas/reasons are accurate.   

The next part of the experiment is to identify the materials.  We record: wood boat, styrofoam boat, aluminum boat, plastic boat, cardboard boat, tub of water, gram weights. 

As students finish copying the materials, I provide each student with a copy of the Boat Experiment Procedure to glue into their foldable.  The ESE/ELL students already have this but I provide it to all students to save on time.  There are several steps to completing the experiment and if we recorded and discussed each one, it would take the entire class time.  By providing them the copy, they can focus on me as I go over each step instead of writing as I am going over them.  As I go through each step, I stress things that are control variables, and model what each step will look like.  Once they begin the experiment, they test independently so I want to make sure they understand each step.  I also use this time to make the connection to the standard on gravity.  I ask students how adding weights to the boat helps the force of gravity overcome the force the buoyancy, eventually causing the boat to sink.  They are able to tell me that adding weights increases the amount of matter that gravity is pulling down on.  

After going through the procedure, we create our data chart in the center of the foldable.  The chart has 6 columns with the headings Trial 1, Trial 2, Trial 3, Trial 4, and Trial 5.  Each group will not be testing each boat 5 times, instead we will be sharing data at the end of the experiment so that each groups data will count as one of the trials.  The data chart also consists of 6 rows titled, Styrofoam Boat, Plastic Boat, Cardboard Boat, Wood Boat, and Aluminum Boat.  


After we go through the procedure, we record the variables on the outside of the foldable.  

  • Independent Variable - The material of boat 
  • Dependent Variable - How much weight each boat can hold 
  • Control Variables - The size of the boat, the amount and type of water, the tub being used, the placement of the weights, the weights being used 



20 minutes

Conducting the Experiment 

My original plan for this experiment was to set up stations and have groups rotate around so that each group is testing one boat at a time.  I quickly learned that I did not have enough weights to do it with station rotations.  Instead, all students form a circle around the front table and we conduct the experiment whole group.  If you have enough weights, doing this in station rotations is a much better strategy.  By rotating around, there are only 3 - 4 students testing each boat at a time so everyone gets a chance to participate.  Doing it whole group means only 1 out of 20 students is actively doing something while the other 19 are watching.  

I continuously remind students that I am choosing quiet students who are doing the right thing to come up and add weights to the next boat. By choosing only quiet students who are focused on the activity, it encourages them to participate and remain on task.  I encourage them to help count as the person is adding weights so they feel part of the test.

As you can see from the video of boat experiment for the winning boat, each boat help a lot of weight.  This boat held over 1,000 grams of weight.  Students did have to be careful of how they were placing on weights which is a variable we discussed before starting the experiment.  You can see in the video of boat experimenting where boat tips over that if the weights are not added in the right place, to balance the boat, it will tip over.  



Wrap Up

10 minutes

Sharing Data 

After we complete 3 tests with each boat (since we ended up doing it whole group instead of rotations we only completed 3 trials instead of 5), all students return to their seat.  We go through each trial to analyze the data.  We look for data that does not fit the trend such as the aluminum boat in trial 1 which only help 150g and all other trials for aluminum held over 380g.  After throwing out that data, we determine which boat held the most weight prior to sinking for each trial.    


I was happy to see that most of our data came out pretty accurate and consistent.  The aluminum and cardboard boats held the least.  The wood and styrofoam held the most with wood holding slightly more than the styrofoam.  The students who selected styrofoam for their hypothesis were really cheering on that boat for the last trial.  They wanted to get it up to more than the wood but it did not happen.   

Students use the data collected to create a bar graph for their homework.  We share some of the graphs the following day.  


After we review the graphs, I ask students questions to relate the activity to our unit on forces.

  •  What force is pulling the boat down against the water?  Gravity
  • What force is acting against gravity, keeping the boats floating?  Buoyuncy
  • Are the forces balanced or unbalanced before we add weights? Balanced  
  • What happens as we add weight to the boat?  The force the boat is applying increases until the boat sinks.  
  • How do you think experiments like this regarding buoyancy have helped scientists? One student says large cruise ships have air pockets in the bottom of the boat that help it float.  Another student says the material they use to make pool toys are styrofoam and plastic, not wood or aluminum.