Dtheta / dt?

1. Oct 11, 2009

emyt

1. The problem statement, all variables and given/known data

just a conceptual thing I don't quite understand..
how come dtheta/dt = v/r? I thought it was basically the same thing as v = r/t, but with radians..
thanks

2. Relevant equations

3. The attempt at a solution

Last edited: Oct 11, 2009
2. Oct 11, 2009

rock.freak667

Re: dtheta/dt?

ω=dθ/dt

now θ is the angle of an arc of a circle of radius r and arc length l.

so θ=l/r

So ω=d/dt(l/r)

r doesn't change so
rω=dl/dt

and dl/dt=v

so rω=v

3. Oct 11, 2009

emyt

Re: dtheta/dt?

Hi, thanks a lot for the reply. Do we say θ=l/r because the arc length might not be in radians?

thanks

4. Oct 11, 2009

rock.freak667

Re: dtheta/dt?

Are length is defined as l=rθ when θ is in radians.

5. Oct 11, 2009

emyt

Re: dtheta/dt?

EDIT: oh never mind, I've got it.. I've somehow forgotten that a radius could be more than 1 unit :P

thanks!