So today we're going to be making something which has
been called the Bernoulli cups.
Whether that's an appropriate name or not is something that
I won't comment on.
All we need for it are these two polystyrene cups that I'm
going to arrange end to end somehow.
And I'm just going to tape them together around the
middle, like so.
A little bit more, maybe.
Right, that should do the job, apart from the couple of
elastic bands.
These are probably not quite long enough, but we can do
something about that.
We'll just fold one through the other and make a loop,
double-length elastic band.
That's all we need.
So let's go and find out what this does, for which we'll
need a bit more room, I think.
OK, we're in the gallery of the lecture theatre, which
obviously is the best place for launching flying objects,
which hopefully is what this is about to become.
Well, first of all, I have to just wind the elastic band
around the middle, and hopefully wind it on itself so
it doesn't pop out.
I wonder if I'm doing it the right way.
I can't really remember what's the best way of doing it.
And then I'll hold on to that a bit, hold
it a bit like that.
And, mm-hmm.
Mmm, yeah, flying would be a generous interpretation for
what just happened.
Oh, yeah, that's better.
You sort of saw it loop up a bit before it ran out of steam
and then kind of plummet.
Well, luckily, I've got plenty more, so we can
have a few more goes.
Ooh, a bit of a misfire.
So what I think we saw there was - well, first of all, the
cups fell a lot more slowly than you
might think they would.
They drifted down pretty slowly.
But also, I don't know if you saw at the start, when the
cups were moving quite fast, they sort of curved up a bit
before drifting back down again.
So there's obviously some effect going on there.
Now, aerodynamics is pretty fiendish to explain.
And getting two people to agree on the best way of
explaining something, it's as likely to end in a fist-fight
as not, I think.
So I'm going to tread quite carefully.
But one of the things that I think is going on here is
something called the Magnus effect.
And this applies to objects that are spinning as they move
through the air.
So it applies to footballs when - this is what makes
footballs curve when the players
apply spin to the ball.
So with the cups, as they move, they're sort of spinning
backwards like this.
And you get sort of air entrained around the cups,
spinning with them.
And that sort of creates a higher air pressure area
underneath the cups, and sort of lower pressure on top.
And that supports some of the weight of the cup, even all of
the weight of the cup at the start.
And you get this sort of kicking up like that, and then
just gradually slowly drifting down.
And then eventually the spin runs out of oomph, and it just
falls on the ground.
This is pretty easy to make, so have a go yourself.
See what you think.
And see if you can come up with a better explanation than
I have given.