This is an issue that has come up many times. The confusion goes back to when 1 Hz was considered to be the same as 1 cycle per second, and a cycle was usually thought of in terms of a rotating element in some application of mechanical engineering. However, 60*rpm is an angular velocity, not a frequency, and Mathcad therefore converts it to an angular velocity of 6.283 radians per second but unfortunately does not show the radian as a unit. There are some old very long threads on this topic. Here's one of them:
http://communities.ptc.com/message/53939#53939
It's an old thread so some of the thread order got mixed up when the forums were changed, but it's so long I doubt you want to read most of it anyway 
. The first few posts will give you an idea of where the rest is headed (into a very long argument!). And it's only one of several!
Mathcad can deal with this though. It has an additional unit, Hza, which will give you the answers you want.
