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A linear elastic strip of natural length a and stiffness k lies between x = 0 and x = a. Each point on the strip is transformed by a differentiable, monotone increasing function f.
a) Characterise the change in potential energy.
b) Given the boundary conditions f(0) = 0 and f(a) = b, choose f such that the potential energy is minimised.
My first thought was to find a piecewise linear approximation to the problem and then take the continuum limit.
If we let [itex]x_0, x_1,..., x_i,...,x_n[/itex] denote an ordered set of points joined by springs then we have [itex]\Delta{E_i}=\int\limits_0^{e_i}\! kx + k(x - e_{i - 1})\, \mathrm{d}x=k(e_{i}^2-e_{i}e_{i-1})[/itex] where [itex]e_i=f(x_i) - x_i[/itex] and [itex]\Delta{E_i}[/itex] denotes the change in potential energy associated by the displacement of [itex]x_i[/itex] given that [itex]x_i[/itex] is displaced after [itex]x_{i-1}[/itex]. We then have (neglecting the endpoints) [itex]\Delta{E_{total}}\approx\!k\sum\limits_i\!(e_{i}^2-e_{i}e_{i-1})[/itex], but I am not sure where to go from there.
Any help would be appreciated.
Edit:
If we factorise [itex]\!k\sum\limits_i\!(e_{i}^2-e_{i}e_{i-1})[/itex] to give [itex]\!k\sum\limits_i\!e_i(e_i - e_{i-1})[/itex] then in the limit we get [itex]\Delta{E_{total}} = \!k\int\limits_x\! e\mathrm{d}e=\frac{ke(x)^2}{2}\bigg|_{x_0}^{x_1}[/itex], but this lack of dependence of internal state runs counter to intuition; it seems to me that if you hold the ends of a rubber band fixed and pull the middle to one side it will snap back. Have I done something wrong? If so, what?
a) Characterise the change in potential energy.
b) Given the boundary conditions f(0) = 0 and f(a) = b, choose f such that the potential energy is minimised.
My first thought was to find a piecewise linear approximation to the problem and then take the continuum limit.
If we let [itex]x_0, x_1,..., x_i,...,x_n[/itex] denote an ordered set of points joined by springs then we have [itex]\Delta{E_i}=\int\limits_0^{e_i}\! kx + k(x - e_{i - 1})\, \mathrm{d}x=k(e_{i}^2-e_{i}e_{i-1})[/itex] where [itex]e_i=f(x_i) - x_i[/itex] and [itex]\Delta{E_i}[/itex] denotes the change in potential energy associated by the displacement of [itex]x_i[/itex] given that [itex]x_i[/itex] is displaced after [itex]x_{i-1}[/itex]. We then have (neglecting the endpoints) [itex]\Delta{E_{total}}\approx\!k\sum\limits_i\!(e_{i}^2-e_{i}e_{i-1})[/itex], but I am not sure where to go from there.
Any help would be appreciated.
Edit:
If we factorise [itex]\!k\sum\limits_i\!(e_{i}^2-e_{i}e_{i-1})[/itex] to give [itex]\!k\sum\limits_i\!e_i(e_i - e_{i-1})[/itex] then in the limit we get [itex]\Delta{E_{total}} = \!k\int\limits_x\! e\mathrm{d}e=\frac{ke(x)^2}{2}\bigg|_{x_0}^{x_1}[/itex], but this lack of dependence of internal state runs counter to intuition; it seems to me that if you hold the ends of a rubber band fixed and pull the middle to one side it will snap back. Have I done something wrong? If so, what?
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