Plummeting Teddies

Not a Biological Burbling this week, but a Physical one. What shall we call it? Physics is not my strong point. Physical Fumblings, perhaps? Hmmmm, maybe not.

Anyway, I’m teaching Year 8 Science. Each term has a topic and we’re half way through Forces….

Now I never got on with Forces when I was at school. I hated the apparent abstraction. Worse, I hated the fact that I didn’t understand what was going on. Sure, you can feel the force of gravity pulling a brick down, but no-one could/would explain how a stationary, inert table exerts an upward Force. Where has this Force magically appeared from? It wasn’t there when the Brick was off the table. And even quite recently, before I had to immerse myself in all of this stuff, I got quite cross and frustrated because a moving object was said to have balanced Forces – which just didn’t make sense to me.

It seems I’m not alone. From talking to the Head of Physics and others, from teaching this topic last year, it’s clearly a conceptual problem that lots of students have.

So when I sat down to re-write the KS3 SoW for Years 7&8, I had to get my head round it – after all, you can’t teach anything effectively if you don’t understand it. But also, having an insight into why students might find it difficult is vital for planning an effective teaching strategy. Indeed, if an idea or concept is so blindingly obvious to you that you can’t comprehend how someone else might not understand it, you’re unlikely to make a very good teacher. And if you’re what I now call an Ocado Teacher – someone who confuses Teaching with Delivering information – then the problem doesn’t even exist.

Last year, when I did this for the first time and was still feeling my way, I tried loads of stuff that didn’t work. Pebbles dropped into cylinders of water didn’t work as a way of investigating/understanding how Forces change on a descending object. I wanted something more interesting, more relevant, more fun. And I wanted it to work.

What I felt was needed was to take things right down to basics and provide the underlying mechanisms. Wind resistance, for example, is a familiar Force, we can feel it, name it, but if all we do is give it a name and an arrow then we’re not helping students understand what it is or how it might vary, other than with some statement like, “the wind is blowing harder”.

This was clearly a job for Cuddly Ted!

Actually, I ended up using my prized collection of fluffy birds, as we hadn’t had time to put in an order for 30 or so small teddy bears. Bear (ho ho) with me…

Context – we had covered some introductory stuff on different types of forces. They had carried out experiments with slopes and surfaces and sails, to demonstrate gravity, friction and air resistance. This was mainly about experimental design, but  we had discussed, briefly, the idea that a bigger sail has greater air resistance because it’s bashing into more particles in the air. We look at the video of a Physicist shooting himself under water. This is very cool.

Why isn’t he dead? Why does the bullet stop???

We’ve also drawn some Forces arrows. And we’ve done some Galilean “gedanken” experiments to think about moving objects in the imaginary absence of friction…

slopes-and-surfaces-3

Now I wanted to focus on what happens when you drop an object from a height – what changes and, crucially, why?

A small bear/fluffy bird, dropped from the top of the Physics fire escape staircase falls, with obvious predictability, to the ground. The class is very happy with the idea that it is being pulled down by a force called gravity (there’s a nice exercise on the Nuffield Physics Teaching site which has students lifting a brick with their eyes closed so they can feel the imaginary elastic band that is pulling everything towards the centre of the earth- they’ve done this in Year 7). They’re also happy that while the bear/bird was in my hand, the Forces were balanced, but that once I let go, and things changed, the Forces had become unbalanced.

At this point the language becomes a bit vague. Why did it fall? Because gravity was pulling down and there was nothing underneath it.

Nothing?

Well, only air.

Only air ????? Is air nothing? Did our bear/bird, now looking somewhat damp and bedraggled (it was a wet day) experience no other forces than gravity on its rapid plummet?

They remember our discussion about air resistance and atoms. The bear/bird is bashing against atoms as it zooms through the atmosphere. They blow on each others faces and experience a trillion zillion molecules bashing their skin…

So how could we save our bear/bird from a grisly plummety type of death?

Of course. A parachute.

Teams of 3. Some thread, a bin liner, some paper clips, scissors and a fluffy bird. Who can make the best parachute?

30 minutes of joyous competitiveness.The Ocado teachers hate this kind of activity – they’re not delivering anything! how can the children be learning if they’re not taking notes???!?!? Well, I could make the cheap point that the students will remember this lesson longer than any note they will ever take. but, of course, it’s all about stimulating the interest and curiosity and enjoyment that will make them want to learn the difficult bit that follows.

So, one by one, the contestants drop their parachuted birds from the staircase and I time the descent. The winner is aloft for over 3 seconds! The loser, well, their parachute didn’t open…

Back inside the lab., we tidy up, spruce up the birds, and then start work on the attached exercise.

sorting-out-forces

What I’m trying to do is provide an explanatory framework for this counter-intuitive notion that a descending object can have balanced forces. It’s structured like a story, but it’s also visual – look, there are the atoms that you bash into when you jump out of a plane. The more you bash into per second, the greater the air resistance…. why might this change…?

Have a go. Tell me what you think. Better still, if you know a Physicist (and most of us probably do) try it on them. What do they think?

For the Year 8 girls, it provoked lots of, “Oh, I see….” which is one of my favourite reactions in teaching. Come the test, will they all draw the arrows correctly….?

I’ll let you know! Back to Bioogical Burbling next week.

 

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