I have to solve following question and am completely stuck. If you click on the link at the end of this post, you'll see a picture for the system. Im my question, the force is acting in opposit direction to that picture.

I found the transfere functionto be : G(s)=1/(cs-k-ms^2) but I have no idea how to find those unknowns.I have to use MATLAB.

Thank you for your help!

Question:
When a unit step function force P(t) is applied to the system, the displacement x(t) has overshoot of a bit less than 5% (indicating that the real part and imaginary part of poles are the same), reaches its final value of 2, at about 4 seconds. From this unit step response of the system, determine the values of m, k and c.

Figure 1: http://wikis.controltheorypro.com/index.php?title=Forced_Vibration_in_Second_Order_Systems

 

Check here:
http://en.wikipedia.org/wiki/Damping

In the section "Example: mass-spring-damper".

 

In M-FILE
function u=unitstep(t)
for i=1:length(t)
if t(i)>=0
u(i)=1
else
u(i)=0
end
end
plot(t,u,'r')
In COMMAND WINDOW
>> t=-10:0.01:10;
>> u=unitstep(t)