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  • [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.

[MICROPHONE FEEDBACK]

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