William W. answered • 06/11/20
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While the object is motionless, the static coefficient of friction will be the "boundary" for which initial motion takes place, i.e., the force required to initiate movement must become larger than μstatic•N to result in a net positive force to move the object.
Once the object is moving, the kinetic coefficient of friction creates the force opposing motion.
William W.
I obviously didn't understand your question. Now, I think I get what you mean. I'm going to assume you mean the wheel is rolling at constant velocity. The reason the contact point has a zero velocity is because of the vector addition of the velocities. There are two velocities involved. 1) the rolling velocity of the wheel and 2) the tangential component of the angular velocity. The rolling velocity is the motion of the center of mass and, if the wheel is rotating CW, would be to the right. The tangential component of the angular velocity and is calculated from the radius times the angular velocity and is directed to the left at the contact point for the wheel rotating CW. They are equal in magnitude but opposite in direction at the contact point so vector addition results in a zero velocity (with respect to the ground). At the point on the opposite side of the wheel (the top), the same thing holds and the vector addition results in a velocity (with respect to the ground) that is 2x the velocity of the center of mass of the wheel. So the velocities vary as you pick different points on the wheel (with respect to the ground) because of vector addition. So, "original velocity" doesn't really have meaning but I understand that you are saying the velocity changes from zero to non-zero. Acceleration of the contact point is associated with the changes in direction (rotational dynamics) not changes in magnitude. Anyway, if the object is rolling WITHOUT slipping, then the frictional force at the contact point is associated with the static coefficient of friction. If the object is rolling WITH slippage, then the frictional force is associated with the kinetic coefficient of friction. I think this is what you are asking.
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06/11/20
Raphael L.
tutor
To further elaborate on what William is saying, if the wheel is rotating without slipping, there is neither static nor kinetic friction. This is because the acceleration at the contact point is perpendicular to the surface of the road (centripetal), not parallel, and static friction would only oppose a parallel acceleration. There is rolling friction, but the typical coefficients associated with this are far smaller than either static or kinetic friction coefficients. William is correct that if the wheel is slipping rather than rolling, kinetic friction would come into play (skid!).
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06/12/20
Zheer 1.
I have heard that an object can't pure roll without static friction and that static friction , is that true?
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06/12/20
William W.
Of course, in order to roll, it must contact a surface. And there are no surface interfaces where the static coefficient of friction is exactly equal to zero. Although there are some that are VERY small (like 0.05) making the force of friction very small. But it’s still bigger than zero.
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06/12/20
Zheer 1.
What do you mean by "While the object is motionless" isn't the object moving but only the contact point of the object has zero velocity? Also this doesn't answer if the contact point opposes kinetic friction force or not.06/11/20