I have a problem where a cylinder is moving on a horizontal surface, starting with velocity $v_0$. It is given that its radius is $10\text{cm}$, its mass is $200\text g$ and the coefficient of friction with the surface is $\mu = 0.25$. I have to find the final velocity.
First I tried to determine torque about the cylinder's contact point with the surface. The normal reaction force and the weight have the same magnitude and arm, but opposite direction, so their torque equals zero. The friction force has no arm, so total torque is $0$. This tells me that there is conservation of angular momentum, $L_0 = L_f$. Since $L = rmv$, and $r$ and $m$ are constants, $v$ has to be conserved so that $L$ can be conserved as well.
This is the conclusion I reached, however, I think I must be missing something, because if there is a friction force, the velocity can't be conserved(?).
I would appreciate any input.