Work-Energy Problem

1. Apr 19, 2012

eurekameh

Is this answer wrong? I think they've made a mistake in the work done by the spring, which I think should be (1/2)(135)(0.5)^3?

2. Apr 19, 2012

Steely Dan

No, the potential energy of a spring is indeed $U = \frac{1}{2} k x^2$. If you want to think of it as work, then know that the spring force is $F = k x$, so the work done is
$$W = \int F dx = \int k x dx= \frac{1}{2} k x^2,$$