Mass Spring system

Let us start with the following example:
The characteristics of this system are the following:
The frequency of the external force has been selected equal to the natural frequency of the system, so that it will be just in resonance. The mass has an initial zero velocity and zero displacement in respect to the equilibrium position. As soon as you click "animation" the mass will start vibrating with a gradually increasing amplitude. As the system has losses (0.01 loss of velocity per simulation step), the oscillations will reach a stable amplitude.

In the next section a brief review of the theory of forced oscillations is presented. There it is shown that the amplitude of the oscillation for the steady state is given by:

So that, in resonance (w = w 0 ) the amplitude should be 2bwa 0 .

To measure this steady state amplitude allow for time being in the order of 10 (for this system) and use the step by step execution until the value of Vx change the sign. You will observe an amplitude (value of x) around 52.

HINT: remember that disabling monitor in the particle inspector window allows a faster execution. To detect the change in sign of Vx you should activate monitor again.

Another important parameter is the phase delay, d, between the position, x, and the acting external force. In the section on "Theory of Forced oscillations", this delay is shown to be 90 when the system is at resonance.

Checking the results

For this experiment we have measured a steady state amplitude of
The theoretical value for A, at resonance is a 0 /(2bw). The value for b in our case is (see section on "Theory of Forced oscillations")

2b = xyzVLoss/xyzTimeStep = 0.01/(1/128) = 1.28

In addition, a 0 = 666.66 and w = 10.00. Substituting in the expression for A we get the value

very close to the measured one.

The phase delay, d, in this case is easily measured to be 90: When the external force is zero the x value is maximum or minimum.

In coincidence with the predicted theoretical value.


Change the loss factor and: