Sunday, 14 March 2010

How many balloons?

Watched "Up" the other night.
First of all, if you haven't seen it, i'd recommend you stop reading this, go out and watch it. Stunning.

Got me thinking about the physics involved, so my opener for year ten this week was "How many balloons do you think it would take to lift you?"

Got the kids to make a few guesses (it was my birthday last week, so we actually had a helium balloon in the classroom - let the kids do a few experiments to test - that balloon could support a pencil and an eraser!)

Then got a few kids starting to ask the important questions:
how much helium can it hold?
how much could it lift?

we took it as a sphere, and found the volume of an average balloon (15cm diameter), converted this cubic cm to litres, before we hit "HowStuffWorks" and found out that helium has a lifting power of 1g / litre.

Now we're cooking!

changed our weights into g (we reckon about 50,000g average per kid?) and worked out how many balloons it would take.

Fun lesson.

Then one kid dropped the question that'd been bugging me.

How many balloons for the house in UP?

Google tells us that the average UK house weighs around 450t, and we work out that it'd take about 30 million balloons to lift one.

Pete Docter (Pixar): " we calculated it would take between 20 and 30 million balloons in real life, we used about 20,000 in the film"

How much do i love pixar that they would take that into consideration?!?!

We then went backwards and worked out how big the balloons would need to be to lift a house if 20,000 of them could lift a house (we think about 3.5m diameter). One of the kids tells me you can actually buy them this size from Army surplus stores in England.

Interesting! The problem i now have is that my kids are determined to buy a load of balloons and orbit the school (we have not considered the cost of helium to be fair...)

Tuesday, 2 March 2010


Asked the kids today what the biggest ever game of dodgeball was.

Took some bets.

Answer? 1200 people, this year in Canada.
Showed them this video:

Talked about assumptions (600 per side?) and then showed them a photo of our gym.
Could we do it in there?

W. asks: is our gym the same size?
Someone tells him not to be stupid, theirs has 2 indoor courts, ours only has 1.

One asks if for the sizes.
Showed them the measurements of the butterdome vs our gym (it should be said that i pretty much made these up based on the measurements of two 5-a-side football pitches).

Kids noticed that the area of our gym is roughly 20 x smaller, so assumed we'd be able to 1200/20 = 60 people in.

C.: But sir, we have like, 80 people in there sometimes for PE.

At this point, i think i'm done, let them run with it.

Some worked out that the record breaking attempt allows for about 8sq metres per player.
Some worked out that there'd be less than 1 player per sq metre.

Question: if we wanted to fit 1200 people in our gym, how many would we need to get in every square metre?

2.5 people per sq metre seems do able - we mark out a square on the floor and try it.

Kids reckon we can fit more in, so we spend a minute seeing how many we can fit in without falling over.

A few point out that we could do this, but it'd be ridiculous - no one wants to be that squashed.

So in pairs we mark out our comfort zones on the floor, and work out the area we'd be happy with.

I pair up with J. and we reckon we'd be ok with about 0.4 sq metres.

We then average this out (it comes to about 0.3 sq metres, apparently i have a bigger social bubble than the kids!) and say, right, if we give everyone this,
how many can we fit in the gym?

Still about 1600 - we could hammer this record, which i sent the guys off to their next lesson (PE) to discuss.