Calculate Force from Energy of a Particle

[ila1985]

If I have a energy of a particle and I need to calculate the expression for force on it how do i do it.

eg: energy of a spring = k*(|x1-x2| - L )^2

where x1 and x2 are particle at end of the springs and K is stiffness. L rest length.

 

[bernhard.rothenstein]

If I have a energy of a particle and I need to calculate the expression for force on it how do i do it.

eg: energy of a spring = k*(|x1-x2| - L )^2

where x1 and x2 are particle at end of the springs and K is stiffness. L rest length.

I think it is better to start with
internal force=-d(potential energy)/dx
potential energy=kxx/2
internal elastic force -kx
In the gravitational field
Energy mgx
Force -mg

 

[ila1985]

Thanks bernhard. What i was wondering is if it is more complicated force like rod bending where energy is given by E = k (theta - original theta)
and new theta is given by tan(inverse) [ (x1-x2)x(x2-x3)/(x1-x2).(x2-x3))]
x -> cross prdt
. -> dot prdt

and X1 and X2 are joined with a rod. and X2 and X3 re joined with a rod. K is stiffness