Refutation Text for the Jelly Chair

I said in Episode 6 I would share what I came up with for the Jelly Chair lesson and refutation texts. I did put some thought in to this but think that there is room for improvement. I ended up writing a few sentences for the kids to do:

  • The ancient Greeks thought that arrows were pushed through the air by the god Apollo. We know this is wrong because…
  • Many people think that objects will slow down and come to rest if no resultant force acts upon them but really…
  • A typical GCSE level understanding of balanced forces allows a person to sit on a chair made of jelly. This is false because…
  • Galileo showed that two balls of different weights accelerate at the same rate when dropped.. Before this people thought heavier objects fell faster but this is wrong because…
  • Some people think the International Space Station has no forces acting upon it because it is in Space, the evidence that they are wrong is…
  • When a helicopter is hovering it is very common for people to think that the weight of the helicopter has an equal and opposite force that is the down-force from the rotors. This is wrong because…

I was particularly happy with the last one (though I have no idea if that is an actual misconception or not), and even happier when one of the students said something like:

The down-force is equal to the weight!

They had truly understood the situation, and was able to explain to their partner that the weight and down-force must be equal in magnitude and in the same direction as weight if the helicopter is hovering (not equal and opposite).

As I roamed the class and spoke to the students I was pretty confident the refutation sentences were having impact. The challenge is to use them again and again and embed them in my practice.

The podcast strikes again. Thank you Ben Rogers!

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Thomas and Robin

6. What happens when your jelly won’t hold your weight? Is it time to lose some mass?

Jelly chair aftermath

Thomas and Robin got together in the same room for a chat this week. Don’t worry, as a good physicist and a good engineer they avoided eye contact*. The big news last week was the redefinition of the kilogram which was originally based on the mass of Napoleon’s leg**, but latterly on a  lump of metal held in what looked suspiciously like a cake container. So we discussed misconceptions about mass and weight and about one of Thomas’ favourite experiments: the jelly chair.

*Well, that’s an unhelpful stereotype!  In fact we were highly empathetic and talked about our feelings extensively.

**completely made up


Now we have listeners, AND an Instagram page (@physics_teaching_podcast) we thought we would encourage you to share the podcast by having our first competition. Win a beautiful podcast T-shirt (in the colour of your choice) by interacting! To win, tell us why you like listening. There are many ways to do this:

Thomas and Robin will pick a winner in a couple of weeks.

It remains an enthralling and inspiring adventure making this podcast for you.  You’ve already taken it in directions we weren’t expecting; it really is your podcast, so please get in touch: teachers of physics are our very favourite breed of hero!

The music we use remains One legged equilibrist polka by Circus Homunculus.

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