May 14, 2010

floating and sinking

After playing with and writing about sinking the boat a couple of weeks ago, I had the idea to investigate what my son understands about floating and sinking by doing an experiment and interview. 

This is something I did during my teaching diploma.  It is helpful to learn what a child understands about scientific concepts so that you can plan experiments to challenge their misconceptions and improve their understanding.

We went around the house and found a variety of objects and set out to test whether they would float or sink.
First we made some predictions.  My son divided the objects into ones he thought would float and ones that he though would sink …
Float – marble, toy pig, plastic spoon, pom pom, syringe, ping pong ball, lid, centre from a tape roll, 5 cents
Sink – golf ball, shell

Then we tested each object and found that two of our predictions were wrong – the marble and the 5 cent piece sink. 

As we tested I asked a few questions. 
Why do some things sink? 
“things that sink are too heavy”

Once we had tested everything we started to experiment with getting floating objects to sink and sinking objects to float.
How could we make the plastic lid sink? 
“make it heavier.  If we put the golf ball in it, it sinks.”
Could we get these things that sink to float?
“we need to pump it up with air or not make it heavier (he means we need to make it lighter but didn’t know the right word).  Put it in a boat.”
Boats are heavy.  Why do boats float? 
“They are pumped up with air”
Scientists use Archimedes’ principle to determine whether objects will float or sink.  Objects are said to be floating if they are supported by the water (that is, they do not have to float at the surface).  For an object that is floating, the mass of the material equals the mass of the water that is displaced by the object.  Dense objects cannot displace enough water to counterbalance their weight.  Objects made of material denser than water (such as a boat) can still float if they contain air so that the mean density is less than that of water.

Some of the earlier understanding that children need to develop about floating and sinking to understand the scientific explanation are …
  • whether something floats depends on the material it is made of, not on its size
  • objects float if they are light for their size and sink if they are heavy for their size
  • an object can be light for its size if it contains air, such as a hollow ball
  • materials with a boat shape will float because they effectively contain air
  • water pushes up on objects with an upthrust force
(This information about children’s early understandings of floating and sinking comes from Ideas for Teaching Science, Floating and Sinking from Deakin University)

So, my son understands that floating or sinking does not depend on the size of the object, but is still coming to grips with whether or not something is light or heavy for its size.  I would like to give him some plasticine to play with in the water (a ball of plasticine will sink, but it can be made into a boat shape that will float), so that he can experiment with changing the shape to make the plasticine float.

Our play with floating and sinking is an example of how play builds children’s understanding.  I doubt my son would understand much about floating and sinking if I had told him, rather than let him experiment for himself (not just this time but many times in his life).

What are your child’s ideas about floating and sinking?  Have your asked them?  Or perhaps you could ask them about another scientific concept.


Sun-Kissed Scholars said...

This is fantastic! I can't wait to conduct our own experiments and do interviews. We did something similiar a few years ago, with my daughter, but have not repeated it with my sons. Sometimes I forget to cover the same things over and over, with each child (sheepish grin.)
Love all the info. I'll link back to you when we try it. :-)

Debi said...

Believe it or not, testing the idea of floating & sinking is something my 5 1/2 year old loves to do in the rain. He seems fascinated by it, actually. And I agree that he's learning far more from this "play" than I could ever teach him with words.