Control system response/MATLAB

This is an EE problem that I'm having a bit of trouble with. It's pretty math heavy and I used to come here with diff eqs help so I thought I would give it a shot.

I have a control system with an open loop transfer function of

G(s) = 1 / s(s^2 + s + 15)

The transfer function describes the input/output relationship of a spring-mass-damper system.

I am simply asked to simulate the step response of the system through MATLAB. Now, I know how to do this, but the response I'm getting on MATLAB doesn't fit what I would expect.

It gives me an almost perfectly linear line, like y = 2x. If I have a spring mass damper system and I provide a step voltage to the motor controlling the mass, I would expect some oscillations in the spring before it settled down at a constant position, but my MATLAB simulation doesn't show this.

I must be missing something. The MATLAB code I have looks like:

num = [1];

den = [1 1 15 0];

sys = tf(num, den);

step(sys)

Which is the correct code, because I've done this stuff for other circuits many times. So I must be missing something about the transfer function itself. Thanks for any help.