- #1
kev931210
- 15
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Homework Statement
I figured out the first part of the question, proving why |t| equals 1, but I have trouble solving the next part of the problem. I expressed F(r(s)) in terms of theta, but I cannot solve for a, b, and c using the equation I derived.
2. Homework Equations
Free energy minimization.
Change of variable for a 2D geometry
The Attempt at a Solution
I first attemtped to convert the given equation F(r(s))= ∫(ds 1/2*k(d2r/ds2)^2) using polar coordinate.
In order to replace (d2r/ds2)^2, I differentiated dr/ds=(cosθ(s),sinθ(s)) respect to s.
(d2r/ds2)^2=(-dθ/ds *sinθ(s), dθ/ds*cosθ(s))^2 = (dθ/ds)^2*sin^2(θ(s))+(dθ/ds)^2*cos^2(θ(s))=(dθ/ds)^2.
Then to replace ds, ds=dθ*ds/dθ.
Eventually I turned F(r(s))= ∫(ds 1/2*k(d2r/ds2)^2,s=0 to L) into F=∫(dθ 1/2*k(dθ/ds), θ=θ(0) to θ(l))
Is this correct?? Well, I thought it was a very simple and beautiful answer, but I could not solve the next problem using this equation.
I do not know how I can minimize F=∫(dθ 1/2*k(dθ/ds), θ=θ(0) to θ(l)) when θ is given as the polynomial equation. By plugging in the polynomial equation,
θ=b*s+c*s^2, a=0 due to initial condition θ(0)=0
dθ/ds==s+2cs
F=∫(dθ 1/2*k(dθ/ds)=F=∫(dθ 1/2*k*(b+2cs)). Then I expressed 'b' in terms of c using the initial condition θ(l)=0;
This is where I am stuck... Could you please help me .. I have been struggling with it all day long.