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I'm trying to prove the following proposition:

The force exerted on the wall perpendicular to the x-axis by a particle of mass m contained in a rectangular box of dimensions a, b, c is given by the negative of the expectation of the derivative of the Hamiltonian wrt a:

[tex]F = -\left\langle \frac{\partial \hat{H}}{\partial a}\right\rangle[/tex]

But I can't see why I should differentiate with respect to the size of the box. Any ideas? This proposition is given in Landau/Lifgarbagez.

Thanks in advance.

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# Force on the walls of a rectangular potential box exerted by a particle inside it

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