Nonlinear second order ODE describing a force field

AI Thread Summary
The discussion revolves around solving a nonlinear second-order ordinary differential equation (ODE) derived from a potential energy curve in physics. The force is defined as F(x) = -dU/dx, leading to the equation x²(d²x/dt²) = - (a/m)b + (ba/m). The poster struggles with solving this ODE and has attempted various substitutions and integration techniques without success. A suggestion is made to apply the chain rule, using the relationship d²x/dt² = v dv/dx, where v = dx/dt, to simplify the problem. The thread highlights the challenges of solving nonlinear ODEs and seeks further assistance from knowledgeable individuals.
thetasaurus
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Not sure if this topic belongs here, but here goes.

Homework Statement



From the AP physics C 1995 test there is a problem that gives the potential energy curve U(x). With F=-\frac{dU}{dx} in one variable,

F(x)=-\frac{a}{b}+\frac{ba}{x^{2}}

Where a and b are constants. Now I need to get x(t)

Homework Equations



Dividing by mass and multiplying by x^2:

x^2\frac{d^2x}{dt^2}=-\frac{ax^2}{mb}+\frac{ba}{m}

Unfortunately I do not have the skills to solve this differential equation.

The Attempt at a Solution



x=y, \frac{-a}{mb}=b, \frac{ba}{m}=k, y'=u

y^2y''=by^2+k

I tried to eliminate the y'':

y'dt=udt

\int{y'dt}=\int{udt}

y=ut+CAnd that doesn't really get me anywhere. Anyone with knowledge of nonlinear ODEs care to help? I tried Wolfram, but even with my Pro free trial it took to much computing time and never gave me a solution.

Also since this wasn't required of the problem per se, and I just want to solve this, I'm not sure what forum it should be in.

Thanks.
 
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hi thetasaurus! :smile:

try the standard trick (from the chain rule) …

d2x/dt2 = v dv/dx (where v = dx/dt)

(btw, that's where 1/2 mv2 comes from :wink:)
 
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