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## Main Question or Discussion Point

Hi!

My question considers no specific problem, but rather different concepts I have trouble getting my head around. So I would be really happy if you could help me understand different kinds of friction, and maybe above all their

Fist we have kinetic fricition acting on an object which is slipping while rolling. This one points in the direction of propagation, thus exerting a negativ torque and slowing the rotation down until rolling without slipping is estabilished. Am I right?

Then there is the rolling friction caused by the surface of the object and ground being not perfectly rigid, resulting in the normal force exerting a negative torque. But can we in this case define any direction of friction?

What I then find really confusing is, for instance, problems dealing with tires moving or accelerating upwars on an inclined plane. In this case (in those problems I have solved), the friction is always pointing upwards along the plane.

An other kind of problem which confuses me is the one where an object starts from rest and is then made to rotate and roll by a friction force, this time directed in the opposite direction, that's in the opposite direction of propagation.

Many Thanks! :)

My question considers no specific problem, but rather different concepts I have trouble getting my head around. So I would be really happy if you could help me understand different kinds of friction, and maybe above all their

**direction**, acting on a rolling object. :)Fist we have kinetic fricition acting on an object which is slipping while rolling. This one points in the direction of propagation, thus exerting a negativ torque and slowing the rotation down until rolling without slipping is estabilished. Am I right?

Then there is the rolling friction caused by the surface of the object and ground being not perfectly rigid, resulting in the normal force exerting a negative torque. But can we in this case define any direction of friction?

What I then find really confusing is, for instance, problems dealing with tires moving or accelerating upwars on an inclined plane. In this case (in those problems I have solved), the friction is always pointing upwards along the plane.

**What kind of friction is this?**I can understand that this force can accelerate the tire upwards if we treat it as a point object, but looking at it as on a rigid body, it seems like this force should extert a negativ torque and slow the rotation down! But in that case the tire could not roll up on the plane... So where does my reasoning fail?**Why doesn't the fricition slow the rotation down?**An other kind of problem which confuses me is the one where an object starts from rest and is then made to rotate and roll by a friction force, this time directed in the opposite direction, that's in the opposite direction of propagation.

**What kind of friction is this?**And**how is the friction directed when an object is simply rolling on a surface without any inclination and why?**

Many Thanks! :)