- [Voiceover] So let's do another curvature example.

This time, I'll just take a two-dimensional curve,

so it'll have two different components, x of t and y of t

and the specific components here will be

t minus the sine of t,

t minus sine of t,

and then one minus cosine of t.

One minus cosine of t.

This is actually the curve,

if you watched the very first video that I did

about curvature introducing it, this is that curve.

This is the curve that I said,

"Imagine that it's a road and you're driving

"along it, and if your steering wheel gets stuck,

"you're thinking of the circle that you trace out

"as a result and it various different points

"you're gonna be turning it various different amounts,"

so the circle that your car ends up tracing out

would be of varying sizes.

So if the curvature's high, if you're steering a lot,

radius of curvature is low and things like that.

So here, let's actually compute it.

And in the last example I walked through

thinking in terms of the derivative

of the unit-tangent vector with respect to arc length

but in this case, instead of doing that,

I just want to show what it looks like

when we take the explicit formula that looks like

x prime times y double-prime,

minus y prime times x double-prime,

and then all of that divided by

x prime squared plus y prime squared.

And then I'm writing x prime and y prime and such

and all of these you should think of as

taking in the variable t.

I'm must being a little too lazy to write it.

And you take that to the three-halves power.

So, this was the formula

and I'm not a huge fan of memorizing formulas

and then hoping to apply them later.

I really do think the one thing you should take away

from curvature is the idea that

it's the derivative of the unit-tangent vector

with respect to arc length

and if you need to, you can just look up

a formula like this but it's worth pointing out

that it makes some things easier

to compute because finding the tangent vector

and everything can be kind of like reinventing

the wheel when you already have the results here.

So, first thing to do is just find

x prime, y double-prime,

y prime and x double prime.

So let's go ahead and write those out.

So the first derivative of x of t

if we go up here that's t minus sine of t.

So its derivative is one minus cosine of t.

And the derivative of the y component

of one minus cosine t,

y prime of t,

is gonna be, derivative of cosine is negative sine

so negative derivative of that is sine,

and that one goes to a constant,

and then when we take the second derivatives of those guys,

so maybe change the color for the second derivative here,

x double prime of the,

so now we're taking the derivative of this,

which actually we just did because

by coincidence the first derivative of x

is also the y component

so that also equals sine of t.

And then y double prime is just the derivative of sine here

so that's just gonna be cosine, cosine of t.

So now, when we just plug those four values

in for kappa, for our curvature, what we get

is x prime was one minus cosine of t,

multiplied by y double prime is cosine of t.

Cosine of t.

We subtract off from that y prime,

which is sine of t,

sine of t,

multiplied by x double prime.

So x double prime is also sine of t,

so I could just say

sine of t squared,

and the whole thing is divided by x prime squared

so x prime was one minus cosine of t,

minus cosine of t,

squared, plus y prime squared

so y prime was just sine,

so that's just gonna be sine squared of t.

And that whole thing to the power three halves.

And that's your answer, right?

You apply the formula, you get the answer.

So for example, when I was drawing this curve

and kind of telling the computer to draw out

the appropriate circle, I didn't go through

the entire "find the unit tangent vector,

"differentiate it with respect with arc length" process,

even though that's decently easy to do

in the case of things like circles or helixes

but instead I just went to that formula.

I looked it up because I had forgotten

and I found the radius of curvature that way.