[MICROPHONE FEEDBACK] -Is this thing on? [TAPPING] [COUGHING] -So a man walks into a bar. He asks for 10 times more drinks
than everyone else. The barman says, now, that
is an order of magnitude. MATT PARKER: Actually quite a
good joke, because order of magnitude is how big
a number is. The speed of light is 3 times 10
to the 8 meters per second, and that's the order
of magnitude. It's 3 with eight
0's after it. And something like a kilometer
is 10 to the 3 meters. So light in one second goes
five orders of magnitude further than a kilometer. And so when he says, I want 10
times as many drinks, he's actually ordering one order of
magnitude more than anyone else, which is, of course,
an order of magnitude. -So an infinite number of mathematicians walk into a bar. The first one, he
orders a pint. The second one, half
a pint, then a quarter, then an eighth. Eventually, the barman hands
over two pints and says, you mathematicians. You just don't know
your limits. MATT PARKER: OK, so if you start
with 1, and then you add 1/2, and then you add 1/4, and
then you add 1/8, and each time you're getting smaller
and smaller, what we're actually doing is we're
summing all the 1/2 to the n's-- some n. And we start with n equals 0,
and we go all the way up to a correctly drawn infinity sign. And if you carry this on
infinitely, its limit equals 2, which is why the barman
gave them two pints. And he said, you don't
know your limits. Because that's the limit. -So why is 6 afraid of 7? Because 7 ate 9! MATT PARKER: Not strictly
a math joke, but OK. The thing here is that numbers
appear in order. They go 1, 2, 3, 4,
5, 6, 7, 8, 9. Why is 6 scared of 7? Well, when you say 7, 8, 9, the
word "eight" in English sounds a bit like the
word "ate." So it sounds like 7 ate 9. So eight is, in this
case, a word. And 7 then eats-- I don't know. I mean, I personally would say
that 7 is a 6 offender, but I'm not the one doing the
jokes here, am I? -How do you make seven even? Remove the S! MATT PARKER: I was promised
number jokes, but I guess another English one is fine. So the number 7-- if you write it out in this
language called English, which you may have heard of, you spell
it "seven." And then if you remove the S, you're left
with the English word "even." No actual math's involved. -Two cats are standing
on a roof. Which one falls off first? The one with the smaller mu. MATT PARKER: OK, bear
with me here. I'm going to represent
the roof like this. The front door and chimney
are optional. And then I'm going
to approximate the cat as a rectangle. Gravity is forcing the
cat directly down. But of course, it can't go
straight down because it's on a sloped roof. And so you're going to split
this into a normal vector, which comes off this way,
and then the vector going down the roof. What stops it from falling
off the roof is friction. And friction will be a force
coming back this way. And friction is proportional to
how hard the box is being pressed down into the roof. And so we have the coefficient
of friction, which is the Greek letter mu, times whatever
that normal force is. And so the bigger the
coefficient of friction, the less likely it is to fall off. And if there were two boxes,
the one with the smaller coefficient of friction
would fall off first. And that is the Greek letter
mu, which, if we're talking about cats, the cats make
the sound meow. -What did the number 0
say to the number 8? Nice belt. MATT PARKER: OK, I guess so. So 0 looks a bit like this. And if you imagine putting
a belt around 0 and then tightening it, it would
squeeze the middle of 0 in like that. And if you tighten it enough,
it would pinch off and would form an 8. And so an 8 is a bit like
a 0 wearing a belt. Why it is a nice belt,
I'm not sure. [MOSQUITO BUZZING] -What do you get if you
cross a mosquito with a mountain climber? Nothing. You can't cross a vector
and a scalar. MATT PARKER: Ok, there's
a lot going on here. We'll start with what
we mean by cross. And it's a way of
saying multiply. When you multiply numbers
together, it's easy. There's one thing you do. You multiply one by the other. When you multiply vectors, it
gets a bit more complicated. So let's say I've got one
vector u, which has ux component, uy component,
and uz component, because it's in 3D. And then I've got another vector
I'm going to call v. So I've got vx component, vy
component, vz component. And then I want to multiply
these two vectors. And there's more than
one way to do that. You can dot multiply, which
gives you a scalar, which is just a normal number,
out as an answer. Or you can cross multiply-- using our traditional cross
multiply symbol-- which gives you out
another vector. Now, cross multiplying is a
little bit complicated. OK, let's do the first one. So it's uy normal multiply vz
minus uz normal multiply vy. Then the next component--
and you keep going. And you get three new components
out this way. The trouble is, you can only
cross multiply vectors. You can't do the same thing
for normal numbers, or scalars, as we call them. And someone who climbs the
mountain is scaling it. So I guess you could
call him a scaler. And a mosquito-- we're going right outside maths
here-- a mosquito can transmit diseases, which in
biology, you would call a vector as a way of transmitting
diseases. And so you can't do a cross
multiplication between a vector and a scaler. And so that's the joke. -Did you hear the one about the
constipated mathematician? He worked it out
with a pencil. MATT PARKER: No. We're aware that there
are other maths jokes we've not covered. We're also aware that some of
you have discovered the comments section underneath
YouTube videos. So if you have your own maths
joke you'd like to contribute, we'll not stop you. It's definitely going
to happen. And we will endeavor, if we get
enough interesting ones, to explain the maths behind
more maths jokes.
