Preparing for Robotics

Preparing for Robotics
Students at DC's Whittier Educational Campus with ReSET Volunteer Peter Mehrevari

Thursday, January 26, 2012

ReSET Volunteer David McInnis on Floating and Sinking with 3rd Graders -

This week I was on my own; two classes, about 1 hour each.  That’s about 35 3rd grade Tasmanian devils swirling around with unlimited energy in a small classroom vessel.   The lesson is what I assisted with last week in the 4th grade classes, sinking and floating, aka density.
I dressed up nicely (stopping short of a tie which should only be used for weddings and funerals) , used real anti-antiperspirant, and had a cup of chamomile instead of coffee.  I gave Ms. Haynes a quick warning that this was my first time teaching elementary students and that I was a bit nervous.  She patted my shoulder and with a huge smile declared, “Don’t you worry.  I’ve got your back.”  I felt grateful and relieved.
After being introduced and unpacking the supplies, I started the “Floating and Sinking lesson” lesson, a ReSET standard.  This lesson involved using several containers of water and definitely required a stack of cleanup sponges.
The point of the lesson was to act as a demonstration of the scientific method and as an introduction to what density is.  In last weeks 4th grade lesson we actually weighed a baseball and a golfball.  The 3rd graders haven’t quite learned to read scales so I used a simple balance made of a clothes hanger with sandwich baggies attached at opposing ends.   As a class, we guessed and then measured how many golfballs equals one baseball (3).  I then gave them 150g as the weight of the baseball and asked them to find the weight of one golfball.  As it happens this lined up perfectly with their current math lessons and Ms. Haynes jumped in, taking over and giving me a much needed break.  I was sweating up a storm, my thoughts were jumbled, and I had no idea where we were going next.   Luckily I had written down a simple check list of the activities and located where we were.
Next we formed a hypothesis about what floats and what sinks in water.  They had some wild ideas but generally agreed that heavy things will sink and light things will float.  No one in either class voiced dissent, but I felt a little guilty corralling their ideas toward this conclusion.    We then placed both balls in a large container of water and made an observation…     the heavy item floated and the lighter sunk.
There were 4 tables, with 4 to 5 students at each.  They shared the equipment.   After a few steps chaos was starting to erupt over who was doing what.  It was critical to set up an order of turns; I simply numbered  around each table.
Here I tried to explain density and stumbled, wallowed, and was left facing a room full of puzzled faces.   At this point the teacher rescued me and held up two clear plastic containers holding markers.  One was nearly full, the other only had a couple of markers in it.  She explained that density was like having more stuff packed into a space.   So which of the containers had a ‘high’ density?
Next we tried floating a lacrosse ball in a cup of water and observed that it sinks.   Then, after removing the ball, we added about 1/4 cup of salt to the water.
Several of the kids in both classes had a taste of the material before being told what it was.  Sigh.  So, yes, third grade still requires purely non-toxic materials.
After adding the salt, we placed the lacrosse ball back in the water, where it floated!  Sounds simple, right?  The students went crazy over this.  After things calmed down a bit, we talked about how we had changed the density of the water.
For the final experiment we took some of the salt water and dyed it red…  well, I dyed them red.  When then took a clear cup of water and used an eyedropper to gently place some salt water along the inside of the cup.   It sinks to the bottom and forms a red layer that the students can easily see.  This experiment turned out to be underwhelming, but they seemed to get the point nonetheless.
In the end I learned that you cannot lecture to third graders and that finding great simple examples to explain you idea is critical.


  1. From ReSET Volunteer Jim Cline:
    I’m not surprised that David had problems with 3rd graders over “density”. Density, being a ratio of two visual entities, involves a concept that many scientists find is beyond most elementary student ability. Years ago the American Physical Society had a contract with NSF to develop science modules for elementary schools in Montgomery County and the consensus was that a density was not feasible for that group.

  2. I don't try to define density with elementary school students, but I introduce concepts of high density and low density and bring in transparent sandwich bags - one filled with sand and one filled with foam packing material. Same size, same shape, different weights. I explain density is a combination of weight and shape.