- #1
guthix
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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
moment of inertia of ball = (2/5)MR^2
The Attempt at a Solution
At first i try to solve the problem by setting up two equations
(1) v = vo - ft/M
(2) w = (5/2) ft/MR
and eliminating t to get v(final)=(5/7)vo
then i thought that if i know nothing about friction, i shouldn't have use "f" as well
and i tried to use another method to solve the problem by conservation of angular momentum
M(vo)R = MvR + Iw
substituting the final condition v=Rw
i still got v(final)=(5/7)vo
but my instructor told me that i can't apply conservation of angular momentum in this case
i was confused, even i use this method, i couldn't explain why this is correct
(i have used this method to solve many rolling problems and it gives at least "correct value of required answer" every time)
can anyone comment on the method i use?