11 million takeover day
lesson
introduction
... from your MC. They must mention how honoured the class are to have for one lesson only this fantastic new teacher (meaning you). They should probably thank the old teacher for letting you take over the lesson (depending on how polite you are).
part 1
Ask the class to sit on one of their hands, so they’ve only got one hand free. Then ask them to clap with the free hand.
Unless they cheat (by clapping hands with their neighbour) they can’t.
So what? They know that a pair of hands is needed. This ‘pair’ idea will crop up again and again.
Now get them to clap, but by only moving one hand. So they hold their left hand still, and move their right hand to make the clapping sound.
Then repeat, but this time hold their right hand still and move just their left. You should again hear the lovely sound of applause.
So what? Again you’ve shown that so long as you’ve a pair of hands you can clap. The science to mention is that the two hands mean you have a ‘pair’ of forces.
Lastly, before everyone is fed up of clapping, get them to repeat the clapping holding one hand still trick, but this time clapping pretty hard- enough to make it sting a bit, but not hard enough so they break bones. They should find that it didn’t matter whichever hand they kept still, the stinging is felt on both hands- again showing the pairs of forces idea.
Have your assistant show a forcemeter to the class. (Forecemeters are sometimes called spring balances or Newton meters). Hook it around the leg of a stool or chair and pull gently, horizontally. Get someone to read the force. Then ask what the other force is, and how much. Depending on how patient you are, you’re likely to get pupils mentioning weight, friction, all sorts of stuff, but they are unlikely to provide the answer you want which is that the stool leg is pulling back with a force equal to the one you measured on the forcemeter. At this point, some pupils will wonder how this can be true.
You can convince most of them by looping two forcemeters together so you measure the forces at the stool end and your hand end. Have a volunteer read the meters. They’ll show identical numbers, but each half of the pair’s pulling in the opposite direction.
Now it’s time for your volunteers with the roller skates or skateboards. Have your volunteer stand facing someone else. (Be safety aware- be sure the area around them is clear). They should have practiced this a few times before. The first time, the pupil or the skateboard is pushed backwards by the person standing (pushed hard enough to move them a little way). Your pupils won’t be too amazed by this, but ask them to consider if the pairs of forces idea is still alive and well.
If the person on the skateboard does the pushing now, the same thing happens. Check with your skateboarder that they pushed forwards. Why did the skateboarder roll backwards? It was the paired force from the person standing on the floor.
If the skateboarders face each other while each standing on their board, and push, they each roll backwards. A perfect proof of the paired forces idea, pushing one way and being rolled back the opposite way. (Safety again: be sure there’s room behind each skateboarder to roll back- and be sure to have practised a few times beforehand, so they can judge how hard to push!)
Bonus question… Ask the class why if the pairs of forces could push each skateboarder in the final skate demo, why didn’t we notice them in the first two demos, where one of your helpers was just stood on the floor? (The answer is that although the force was there, it wasn’t strong enough to move the person standing firmly on the floor. Wheels make things easier to move by reducing friction- less force is needed to push something with wheels on).
part 2
You’ll have to see how the time’s going at this point. Pick and choose from the following:
Show one or two scicast film clips showing forces at work.
Show a film clip of a gun recoiling after it’s been fired, or show a rocket being launched.
Do the balloon on a wire trick.
Blowing ball into a bottle trick.
Go outside and launch a water rocket.
finishing off
Try to have a few minutes at the end to remind your pupils what they’ve seen and hopefully learnt.
Forces come in pairs - pushing no use if you’ve nothing to push against!
The paired forces go in opposite directions to each other.
The forces have equal strengths.
You’ve just helped your class wise up to a famous law of motion, from a famous scientist. The homework will help your class find out what law and who the scientist was.
Run off copies of the ‘pairs’ crossword for as many people as there are in your science class. (You too, and your teacher - answers will be available on the day).
You can reveal the answers in your next lesson, so people can check their homework themselves.