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It is clear that if a frictional force must exist on an object that rolls without slipping, it must a static frictional force because the contact point of the object with the ground does not move.

I know that if a ball rolls on a flat surface, once it begins rolling without slipping, the frictional force is 0. I am confused, however, about the case when a ball rolls down a ramp. For the condition of rolling without slipping to hold, there must be a frictional force on the ball up the ramp. I am confused about where this frictional force comes from if the part of the ball touching the ramp isn't actually moving. It doesn't seem to make physical sense because from the ramp's perspective, it is touching a stationary object and thus there shouldn't be a frictional force.

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Static friction force arises whenever there is interaction between two bodies by direct contact (touch). There need not be any mutual motion between the bodies.

This friction force is necessary to explain why the bodies around us maintain their position so reliably. Without friction forces, there would be nothing opposing their mutual motion and the world would look more like a floating cloud of debris than like a solid body.

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  • $\begingroup$ why is there a frictional force when it rolls down the ramp and not when it rolls on a flat plane???? $\endgroup$
    – curiousgeorge
    Commented Sep 1, 2014 at 19:18
  • $\begingroup$ The friction force is present whenever the conditions require it. On a flat plane, the idealized conditions are such that the ball can roll with no slipping uniformly, so no force is required. On an inclined plane, the ball has to accelerate and to do so with no-slip condition, force is required. The ground will act with friction force of magnitude and direction necessary to maintain the no-slip condition, unless the required friction is higher than the contact can transmit, at which point slip occurs. $\endgroup$
    – Ján Lalinský
    Commented Sep 1, 2014 at 20:15
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Friction is always there, been proportional to perpendicular component of weigh to the plane, without friction the ball does not rule. It would roll in the same position no moving the center of gravity in the case of flat plane, in the case of incline plane the ball would move down on the plane but no torque no roll, momentun cero. When speed increases enough , the ball can start to slip,and the máximum friction force keeps constant proportional to the weight component vertical to the plane.

But you are not using twice the friction Force, only once to produce the torque, you have the gravity Force and the reaction Force of the plane as well as the friction, result is movement in the line of the plane and rotation. No friction no torque, no roll.Friction produce the torque.

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  • $\begingroup$ "the ball does not rule"? $\endgroup$
    – Danu
    Commented Sep 28, 2014 at 15:33
  • $\begingroup$ I understand that it's difficult for non-fluent English speakers to participate sometimes, but you will really have to improve your grammar and spelling in order to make this answer comprehensible. $\endgroup$
    – Danu
    Commented Sep 28, 2014 at 15:34

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