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biki is wrong. In both the cases, friction will act upwards. This is because of the following:
1. When the disk is rolling down, we all know that friction is an opposing force, so the direction of frictional force is opposite to the motion of the disk, ie, it is upwards.
2. Here's the catch, when the disk is going up, Friction helps the disk in the motion, so the direction of friction is taken as upward.
Just think, if there were no friction, would you be able to roll a ball up an inclined plane? No! It is friction which helps the upward motion (Rotation+translation) of any object.
Therefore, in general, friction aids upward motion. You will come across this concept quite often when you solve inclined plane problems.
Cheers!
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Moderation Team
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