# Question about units for angular velocity, time constant

1. Jul 25, 2011

### ahunter10

Here is a link to page in a book which contains an example problem:

http://imgur.com/OPrlw.jpg"

In the book, they work out the natural frequency of a hydraulic cylinder and come out with an answer in rad/sec. This number is then inverted to get a time constant, and the resultant unit is seconds.

I understand that a radian is dimensionless, and 1 rad/sec really equals 1/sec. So, it makes sense that you invert it and get seconds. However, you would also get seconds if you first convert the frequency from rad/sec to cycle/sec, and then invert.

My question is: how do you know which to use? When do you want to use sec/cycle, vs. sec/rad? It seems ambiguous, and the numbers would come out very differently.

I know the result of this equation is in radians. What if you experimentally measured the natural frequency in cycles/sec, and then inverted to get the time constant in seconds? You would get a different answer, but I don't think anything was done wrong.

Can anyone shed some light on this? I think I am missing something.

Last edited by a moderator: Apr 26, 2017
2. Jul 25, 2011

### Mech_Engineer

I think you may be confusing a "time constant" vs. a "period." One is a parameter which is indicitive of a system's response, the other is the length of time for one cycle.

http://en.wikipedia.org/wiki/Time_constant

http://en.wikipedia.org/wiki/Period_(physics [Broken])

Last edited by a moderator: May 5, 2017
3. Oct 23, 2011

### valjok

Last edited by a moderator: Apr 26, 2017