Block, Balloon, Brick

 I was inspired by this Darek Dewy Twitter/X post he used to demonstrate the contact forces between two boxes using a balloon. He labeled one box "H" that was "Heavy" and the other "L" for "Light." A constant force was applied by a mass attached by a string over a pulley. You can see the balloon being squished between the two boxes, more so when the lighter box is trying to push the heavier one than the reverse. 

The first time I tried this I planned to demo it. Then I remembered I had plenty of bricks and plenty of blocks. I put a brick and a block on a tray with a partially filled balloon (so that it fits better between the two) and set them out on my student tables. 



I invite students to make a sideways sandwich with a brick on its long side, the balloon and then the block. I ask them to push the system from the block side then from the brick side and make observations. Students experiment for awhile and observe the balloon squish different amounts depending on which side they are pushing. 

As a whole class we discuss which quantities are the same or not the same for each scenario. Students have good discussions about what changes or not for pushing on the different sizes. This relates back to a problem we did earlier about a series of boxes pushing on each other. I do wonder about doing it before but I think it helps to have a bit of background first. 

Above is an example when I was starting to figure out the activity. Since I couldn't reach easily while filming I used a board but now I have students use their palm flat. This video has two bricks so that they are the same height roughly as the block. Now I have students put their brick on the skinny long edge for the same effect and less friction to fight. 



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