Forced Oscillations of Mass-Spring System: Reasons for Observed Behaviour

[nokia8650]

A mass spring system with natural frequency of 1.5Hz is set up as shown:

http://img300.imageshack.us/img300/7922/35809066dx2.th.jpg

Can someone please explain the reasons for the following observations:

When the support rod oscillates at a frequency of 0.2 Hz - oscillations are (almost) in phase with driver

When the support rod oscillates at a frequency of 1.5 Hz - oscillations have a phase difference is pi/2 radians with the driver.

When the support rod oscillates at a frequency of 10 Hz - oscillations are out of phase with driver [or phase lag of (almost) pi on driver]

Thanks

 

[Carid]

Here's a good resource

http://www.walter-fendt.de/ph14e/resonance.htm

Check out the phase difference diagram