**HP Prime CAS: Curvature**

**Introduction**

The following CAS functions calculates the curvature of:

functions, y(x)

polar functions, r(t) (t: Θ)

parametric functions, x(t), y(t)

Let Δα be the angle of rotation angle and Δs is the slight change of distance. Then the radius of curvature is:

K = abs(Δα ÷ Δs) as Δs → 0

And the radius of curvature is the reciprocal of K.

For circles, the radius of curvature is constant. Wankel engines and rotary engines have their pistons traveling in a circle.

Calculating the curvature depends on the form of the function.

Function: y(x)

K = abs( y''(x) ) ÷ (1 + (y'(x))^2) ^(3/2)

Polar: r(t) (t replaces Θ)

K = abs( r(t)^2 + 2 * (r'(t))^2 - r(t) * r''(t) ) ÷ ( r(t)^2 + r'(t)^2 )^(3/2)

Parametric: x(t), y(t)

K = abs( x'(t) * y''(t) - y'(t) * x''(t) ) ÷ ( x'(t)^2 + y'(t)^2 )^(3/2)

Radius of Curvature:

r = 1 ÷ K

For the CAS functions, they take the form:

#cas

name(arguments):=

BEGIN

...

END;

#end

Clicking on the CAS checkbox will not put the #cas and #end delimiters. And these programs will work in CAS mode only.

**HP Prime CAS Program: crvfunc**

**#cas**

**crvfunc(y,x):=**

**BEGIN**

**// curvature**

**// function**

**// radius = 1/curvature**

**LOCAL a,b;**

**a:=diff(y,x,2);**

**b:=diff(y,x,1);**

**RETURN ABS(a)/(1+b^2)^(3/2);**

**END;**

**#end**

**HP Prime CAS Program: crvpol**

**#cas**

**crvpol(r,t):=**

**BEGIN**

**// curvature**

**// polar (t: θ)**

**// radius = 1/curvature**

**LOCAL a,b,n,d;**

**a:=diff(r,t,2);**

**b:=diff(r,t,1);**

**n:=simplify(r^2+2*b^2-r*a);**

**d:=r^2+b^2;**

**RETURN ABS(n)/(d)^(3/2);**

**END;**

**#end**

**HP Prime CAS Program: crvpar**

**#cas**

**crvpar(y,x,t):=**

**BEGIN**

**// curvature**

**// parametric**

**// radius = 1/curvature**

**LOCAL y1,y2,x1,x2,n,d;**

**y2:=diff(y,t,2);**

**y1:=diff(y,t,1);**

**x2:=diff(x,t,2);**

**x1:=diff(x,t,1);**

**n:=simplify(x1*y2-y1*x2);**

**d:=simplify(x1^2+y1^2); **

**RETURN ABS(n)/(d)^(3/2);**

**END;**

**#end**

Until next time and have a great day,

Eddie

Source:

Svirin, Alex Ph.D. "Curvature and Radius of Curvature" Math24 https://math24.net/curvature-radius.html 2022. Last Updated September 12, 2022.

*Gratitude to Arno K. and rombio for helping me with derivatives and CAS programs.*

All original content copyright, © 2011-2022. Edward Shore. Unauthorized use and/or unauthorized distribution for commercial purposes without express and written permission from the author is strictly prohibited. This blog entry may be distributed for noncommercial purposes, provided that full credit is given to the author.