- #1
EEristavi
- 108
- 5
- Homework Statement
- uniform disc of radius R is spinned to the angular velocity
w and then carefully placed on a horizontal surface. How long will
the disc be rotating on the surface if the friction coefficient is equal
to k? The pressure exerted by the disc on the surface can be regarded
as uniform.
- Relevant Equations
- T = Ia
Only problem I have is in calculating Torque
I say:
dT = R dF = R k g dm
&
dm = ##\frac m {\Pi R^2}## R dr d##\theta##
However, in the solution I see that:
dT = r dF = r k g dm
&
dm = ##\frac m {\Pi R^2}## r dr d##\theta##I don't get it: when taking the whole T (when I integrate), why do I have to "Sum" all the Torques along the radius
I say:
dT = R dF = R k g dm
&
dm = ##\frac m {\Pi R^2}## R dr d##\theta##
However, in the solution I see that:
dT = r dF = r k g dm
&
dm = ##\frac m {\Pi R^2}## r dr d##\theta##I don't get it: when taking the whole T (when I integrate), why do I have to "Sum" all the Torques along the radius