Hi,
You can use the online calculator at http://cbmapps.com/apps/34 to do this kind of conversions (provided the vibration is sinusoidal, the theory is in http://www.cbmapps.com/docs/28).
Hope this helps.
Just came to my mind: usually amplitude of a sinusoidal function is taken from equilibrium position to maximum displacement, so it would be 21 degrees. If the small angle approximation holds (usually you can assume it holds for amplitudes below 45 degrees) the angular position of the boy could...
In short, assuming pure sinusoidal vibration at 6.000cpm (pure unbalance, for example):
velocity: 5,642 mm/s rms
displacement: 3545 mm/s2 rms
Rms values are those commonly used in ISO standards, for pk or pk-pk values you would have to multiply by sqrt(2) and 2*sqrt(2) respectively...