wow, thanks very much
(i think) I've understand the problem
just one more minor question
for similar reason, is the total angular momentum of the Earth = angular momentum of Earth's CM about the Sun + angular momentum about the Earth's CM ?
you mean...i can take moment about, say, the initial contact point?
i just think it is difficult to understand, even the line of action of friction passes through it, i can't treat it as no external torque because the point is no longer related to the rotational motion of the ball...
and...
i told him if i take moment at the contact point then there is no external torque, and he ask me whether "that point" is moving or not, i don't know why he ask for this, i have never think about this so i didn't answer.
other classmates challenged me that "as the ball is rolling, the point does...
Homework Statement
A ball of mass M and radius R initially slides with speed vo but without rotating on a horizontal surface with friction.
Without knowing anything about the nature of the frictional force, find the speed of the ball when it begins to roll without slipping.
Homework Equations...