Why Don't They Sink - Countering Density

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Students will be able to show how the density of an object can be countered by completing an investigation.

Big Idea

Why do boats and other large items float when the material they are made of is a high density? This fun investigation helps answer this question!


10 minutes

This lesson is an extension of a previous lesson in which we explored the sinking of the Titanic.  The students began to wonder why a big boat can float.  I decided this would be a great question for us to explore!  The students were so excited about our lesson on the Titanic, I wanted to capitalize on this enthusiasm and also show the students that the world around us can spark questions that we are interested in and want to investigate. 

I found some pictures of some large ships on an image search and displayed them on the Smartboard.  I ask the students, how do these boats float?  The students think and try to express some ideas, but no one can really come up with any type of explanation as to why the ships float. 

I tell the students that we are going to figure out today how these ships stay afloat.  I send the students back to their work stations to begin our investigation.


35 minutes

Materials Needed for Each Work Group:

  • Large container full of water (see photo)
  • Plastic drinking straw
  • 1 snack size zipper bag
  • 1 quart size zipper bag
  • Recording sheet included as a PDF with this lesson
  • About 24 keys, nuts or large bolts

Materials Tip: You will need to do a little testing to make sure you have the right number of items.

The following should happen:

  1. The keys, bolts or screws should sink when placed in the snack-size bag without extra air.
  2. The keys, bolts or screws should sink when placed in the snack-size bag with extra air added.
  3. The keys, bolts or screws should float when placed in the quartz-size bag with extra air added.

*To add air to the bags, place straw in zipper bag and zip bag so strong is sticking out, but the rest of the bag is sealed tight.  Blow air into bag and then slowly remove straw from bag, zipping bag closed.  Bag should be inflated like a balloon (see photo).

I pass out the recording sheet to the students and have them write their name on it.

I then tell the students, We are going to figure out why things like large boats can float.  To begin our investigation, I have some keys.  I want you to take a key out of the container and feel and look at it.  Now I want you to make a prediction.  Will the key float or sink?  Make sure you share why you are making this prediction.  Write your prediction on your recording sheet. 

I give the students time to make their predictions.  After everyone has made their prediction, I have one student in each group hold a key over the water.  The class counts off and the key is dropped into the water.  The students record their results on the recording sheet.

We discuss whether this means the key has a high density or a low density (high density).  I point out to the students that many ships are made out of this exact same kind of metal.  Why is it that this little key will sink, but those great big ships float?  No one can explain it yet. 

I tell the class that we are going to figure it out.  I have the student put the keys in the small zipper bag and zip it shut, with no added air. I have them make a prediction as to whether the bag of keys will float or sink and to share the why of their prediction.  They record their prediction on their recording sheet.  After everyone is done, we count off and drop the bag of keys in the water (see video).  The students record the results.

I discuss with them how we might get the keys to float.  There are different ideas, but one student catches on to something (see the video).  We build upon this idea.  I suggest that we add air to the bag by blowing it in through a straw (word of caution--my kindergartners were not able to do this on their own. I ended up having to go around and blow the bags up for the students).

Again, the student make a prediction and we count off and drop the inflated bag of keys into the water.  The students notice that the bag took much longer to reach the bottom. 

I pose a question to the students.  I want to get them thinking about they adjustment that could be made.  I want to help them design the next steps of the investigation. I ask, How can we get these keys to float?  Immediately, the students say, "We need more air!"  But how can we get more air?  The bag is full?  A student comes up with the idea of trying a bigger bag.  The class is excited to try this out. We transfer the keys to the quart bag and again, I blow the bags up with the straw.  The students pass the bag around and make their predictions.  We count off and drop the bag in the water.  The students are delighted that the bag floats.  Now it is time for some discussion. 



10 minutes

Materials Needed:

  • SmartBoard file with cross-section of ship picture

(If you do not have a Smartboard, I included the .jpg file with this lesson)

I gather the students in front of the SmartBoard and we process what we just learned about during our investigation.  I ask them some questions to summarize our learning and then apply what we discovered during our investigation to a real-world situation.

What happened when we dropped the key in the water?

Why did it sink?

How were we able to get the keys to float?

Why did they float?

(I put the picture of the cross-section of the ship on the SmartBoard.)

This is a picture of a part of a ship.  If you cut into the ship and look at what is inside, this is what it would look like.  This is called a cross-section. As I look at the ship, I see lots of spaces or compartments.  These compartments are not full, there seems to be a lot of space in each one (pointing to different compartments).  It looks like there is nothing in this space, but there really is something.  What is in this space?  That's right!

The ship is made out of steel and we know that has a high density.  So, why can the ship float?  That's right, because of the air.  If the ship didn't have these compartments full of air, it would sink.  The air allows it to float.

So, if we want a ship to float, we need to make sure we have lots of air inside it!


5 minutes

To wrap up the lesson, I bring things full circle to help connect today's lesson with the previous lesson about the Titanic.  I ask the following question:

Let's think about the Titanic. When the Titanic hit the iceberg, water rushed into the ship.  What did the water replace?  That's right, air.  Water has a higher density than air and as the density increased, the ship went down.  If the Titanic had hit the ice berg, but it just dented the side of the ship, what do you think would have happened?  Why?