in both cases, i assume two things:
1.) no brakes are being applied
2.)the tyres roll without slipping
a popular misconception is that when rolling, there is a tendency for friction to act in the same direction as the tendency for motion of the center of mass. at a given instant during rolling , the point at instantaneous contact with the ground is at rest**. so friction acts in the opposite direction too. this should answer both your questions, ie.
that when there is no slipping, friction acts along east for both.
** how? well, consider the diagram.
so now you agree that there is no tendency for motion at the instantaneous point of contact to move backwards. this means, FRICTION ACTS BACKWARDS AND NOT FORWARDS
Moderation Team
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