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In my book, there is a diagram in which a circular disc is having pure rolling on ground with a clockwise angular velocity 'w'. If we assume friction is absent, then the point of contact of the disc with the ground will tend to move right to left, so friction should act left to right. But, in the diagram, friction is acting from right to left on the point of contact. Please explain. Suppose I apply some torque to the ball and make its angular velocity w. And then I slowly place the ball on the ground. Then, if friction is absent, wouldn't the point of vontact move from right to left if w is clockwise, then shouldn't the friction be from left to right instead?

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  • $\begingroup$ I saw this exact question a short time ago. I cqn’t spot it to flag as a dup…anyone know where it is? To the OP Dove, browse the friction questions. $\endgroup$ – JDługosz Dec 26 '16 at 5:31
  • $\begingroup$ See posts containing friction direction $\endgroup$ – JDługosz Dec 26 '16 at 5:33
  • $\begingroup$ Possible duplicates: physics.stackexchange.com/q/89209/2451 and links therein. $\endgroup$ – Qmechanic Dec 26 '16 at 6:54
  • $\begingroup$ The direction of friction depends not only on the angular speed of the disc, but also the linear speed. You mention the first is $\omega$, but nothing about the second. $\endgroup$ – BowlOfRed Dec 26 '16 at 7:52
  • $\begingroup$ @BowlOfRed: I was talking about rolling without slipping so obviously, v=Rw. So, when the disc is rolling the velocity of the point of contact is zero. But if frictikn is assumed to be absent, then the point of contact would move from right to left, then shouldn't the friction be from left to right? $\endgroup$ – Dove Dec 26 '16 at 11:23
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Because you need some force to oppose motion of point of contact and make its velocity zero such that it continues pure rolling.

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  • $\begingroup$ Suppose I apply some torque to the ball and make its angular velocity w. And then I slowly place the ball on the ground. Then, if friction is absent, wouldn't the point of vontact move from right to left if w is clockwise, then shouldn't the friction be from left to right instead? $\endgroup$ – Dove Dec 29 '16 at 2:01
  • $\begingroup$ @Dove, You're assuming that you place it on the ground with $v=0$ or $v=Rω$ or something else? $\endgroup$ – BowlOfRed Dec 29 '16 at 3:07
  • $\begingroup$ @BowlofRed: I'm assuming v=0 where v is the velocity of center of mass. Suppose, I slowly place a disc having pure rotational motion on the ground. Then, if the ground has friction, shouldn't the friction act on the disc from left to right, i.e. oposite to the velocity of the point of contact which is right to left due to its clockwise rotation. $\endgroup$ – Dove Dec 29 '16 at 6:54
  • $\begingroup$ @Dove, yes given that specification, you are correct. Your initial question doesn't specify $v$. If $v$ were initially different, friction could act in a different direction. That was why I asked it earlier. $\endgroup$ – BowlOfRed Dec 29 '16 at 7:10

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