Inverse Barbie Bungee

A few weeks ago we were up in Boston for a 25th wedding anniversary party.   We arrived a little early to help set up because I had to leave early for ultimate frisbee practice.    After we finished setting up, a few of the husbands and I a bunch of the kids started playing around with the helium balloons our friends had ordered.  One of the kids wondered how many balloons it would take to lift her off the ground.  This question turned into a quick and fun little experiment that I finally got around to expanding on today.

This morning we ran to Party City and bought 20 helium balloons for the project.  We used three of them to get an empty pop can to float.  Once we got that piece of the project working I brought the boys together to talk through what we’d be doing for the rest of the project and to try to help them think about the things we’d be learning:

Next we headed to the kitchen to start the 2nd part of the experiment:

We continued adding water to the can and tested to see if we needed to add more balloons to keep the can floating.  Here’s an example from when we had 11 balloons and 41 total grams of can + water.

Once we had all 20 balloons tied to the can we returned to the white board to talk through the numbers we collected during the experiment.  One thing we saw clearly was that the results were not linear.  We spent a little time talking about various things that could have caused the non-linearity:

The next part of the project was trying to get “cube bot” to float.  Cube bot is a little toy that they picked up from the gift shop at the Museum of Math in NYC last week.  According to our scale cube bot weighs 38 grams.  Trying to guess how many balloons it would take to get “cube bot” to float was an interesting exercise.  I was pleasantly surprised that my older son gave a range of values for the answer.  Having made some guesses at the required number of balloons, the boys went back to the living room to test out their estimate:

Finally to the question of the day – how many of these balloons would we need to tie to ourselves to float away?  We used 70 pounds as an estimate of the weight of a kid, and then talked through how we could use our data to help figure out the required number of balloons.  Prior to doing the calculation I had both kids give me an estimate of the answer.

This was a super fun project.  The boys really enjoyed testing to see if the can would still float when we added water to it.   The times that the can seemed to be right on the edge of floating and not floating added some excitement to the data collection, too. I was worried that attaching the balloons to the can would be difficult, but it really wasn’t.    I was also pretty surprised that the results weren’t so linear, but that result is a good lesson all by itself.

So, all in all a great day.  Glad we finally had a chance to work through this experiment.

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