In case of rolling friction always tries that there should be pure rolling ..i.e. always v= R...
if the body is moving up the incline then due to mgsin the velocity of body goes on decreasing and there is no force which can increase its velocity therefore friction acts in same direction in which body is moving that is upwards so as to increase v therefore the body can follow v= R.
however if the body is moving down the incline then due to mgsin its velocity goes on increasing and there is no force which has torque about COM.. so friction acts in opposite direction in which the body is moving so as to decrease v and to provide a torque which will increase ... so as to follow v= R.
Moderation Team
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R...
the velocity of body goes on decreasing and there is no force which can increase its velocity therefore friction acts in same direction in which body is moving that is upwards so as to increase v 
R.![[Thumb - friction.PNG]](/upload/2008/2/28/06bb3ae3a274dcfc1aa1ce1cbe9a5891_34466.PNG_thumb)
