- #1
thecourtholio
- 19
- 1
Homework Statement
A particle of mass m moves along the x–axis under the influence of force ##F_x=-ax+bx^3## , where a and b are known positive constants.
(a) Find, and sketch, the particle's potential energy, taking U(0) = 0
(b) Identify and classify all equilibrium points
(c) Find the frequencies of small–amplitude oscillation about all stable equilibrium points, if any
(d) For what range of total mechanical energies will the particle remain bound, executing periodic motion
(e) For what range of total mechanical energies will the particle ultimately escape to infinity
(f) Find the kinetic energy as a function of position in case (e)
Homework Equations
$$U(x)-U(0)=-\int_0^x \vec{F}\cdot d\vec{r}$$
For extrema: $$\frac{dU}{dx}=0$$
Frequency: $$\omega^2=\frac{1}{m}\left . \frac{d^2U}{dx^2}\right |_{x=equilibrium\ point}$$
$$E=T(x)+U(x)$$
The Attempt at a Solution
For (a) I found $$U(x)=\frac{1}{2}ax^2-\frac{1}{4}bx^4$$ and the plot looks kind of like a big M (two peaks, one trough in the middle).
For (b), I found equilibrium points at ##x=0,\pm\sqrt{\frac{a}{b}}##, with ##x=0## being a stable equilibrium.
So for (c), $$\omega^2=\frac{1}{m}\left . \frac{d^2U}{dx^2}\right |_{x=0}$$ $$ \omega=\sqrt{\frac{a}{m}}$$
But now, I'm kind of stuck on part (d). I know that the total mechanical energy is obviously ##E=T(x)+U(x)##, where ##T(x)## is the KE. I think that the particle will be bound between ##-\sqrt{\frac{a}{b}}<x<\sqrt{\frac{a}{b}}##. But I'm a little unsure of how to write this quantitatively. I'm also a little confused as to how to express it as a range of energies.