Sunday, October 13, 2013

MTBoS - Backyard Ice Rink

Each year my husband constructs one of our family's favorite after school spots ... our backyard ice rink.  (It's a Minnesota thing.)  Anyways, each year I pose a very important question to my students,

"How long will it take to flood the rink?"  

First, I have them write down an estimated time in hours.  We talk about how they came up with their estimate and if they need any additional information.  They get out their paper and pencil and start brainstorming questions for me.
  • What are the dimensions of the rink?  
    • 36 x 64 feet
  • How thick should the ice be?
    • at least 4 inches thick
  • What long is the garden hose?
    • 50 feet
  • What is the circumference of the garden hose?
    • 3/4 inch
  • How many gallons come out of the hose each minute?
    • We look it up together here
  • How many gallons are in a cubic foot?
    • We read about it here
This is one of my favorite lessons of the year.  This problem is so rich with math.  Do we ever get the exact answer?  No.  I have done this problem for 3 years with a dozen different Geometry classes and each class obtains a slightly different estimate.  I let my students lead because they ask the BEST questions.  They debate about factors such as weather and time of day.  Some classes add or subtract hours due to weather just as some classes add or subtract hours due to the time of day.  (FYI - We do not get into water pressure and what kind of well we have at my house.  Maybe this year I will check into it.)

It's pretty incredible.  Try it.  See what you come up with!

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  1. Holy crap do I love this!! I built a retaining wall in my back yard and I could bring in a picture of it and do a similar activity with area, cost, time, etc.! What a great idea! Thank you so much for sharing!

  2. This looks like a great problem. I wonder what grade you teach.

    I teach sixth grade, and I think that it might work well toward the end of the year after they have learned about ratios, proportions, rates and a bit of 3D geo. But then I thought maybe it's better to introduce this problem before "covering" the skills needed to solve. Would they still be able to ask all those good questions? It might give them a context for why they needed to learn those skills. When do you do it?

    Also, do you ever just start by asking them about the picture? I wonder if they might come up with your question or something else.

    1. Great idea! I should have them come up with the question. Thanks!

  3. Thanks for sharing! I love messy, real world problems, especially ones students relate to. If I was to share the same situation in Texas, students would probably look at me funny.

    You said no one has gotten the right answer. How close have their estimates been? Have any of them given a fairly reasonable estimate? Has the actual time been consistent across the years? I'm curious about the situation myself! :-)

  4. There really isn't a correct answer because weather conditions do play a role in the process. They usually come up with 24-25 hours. It's pretty accurate when all is said and done.

  5. A related problem:

  6. This problem is fantastic! I'm going to try it out with my advanced classes. I like how there are multiple solutions and that you provided research links. Thanks for sharing this messy and practical question.