Direction of friction between disks

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Discussion Overview

The discussion revolves around the direction of friction between two disks, D and E, in a scenario where disk D rotates with a constant angular velocity and disk E is brought into contact with it. The focus is on understanding the forces at play and resolving confusion regarding the frictional force's direction.

Discussion Character

  • Homework-related

Main Points Raised

  • One participant expresses confusion about the direction of friction, initially calculating it to point left due to the velocity direction at the contact point.
  • Another participant prompts clarification on the nature of the friction force and its reaction force.
  • A participant notes that when isolating disk D, the friction points left to counter the clockwise motion, while isolating disk E leads to the conclusion that friction points right.
  • A later reply confirms the latter interpretation of the friction direction.

Areas of Agreement / Disagreement

Participants appear to agree on the direction of friction when isolating each disk, but there is initial confusion that suggests a lack of consensus until clarified.

Contextual Notes

The discussion does not resolve the underlying assumptions regarding the system's dynamics or the implications of the friction direction on the overall motion.

edsoneicc
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Homework Statement


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Disk D turns with a constant clockwise angular
velocity of 30 . Disk E has a weight of 60 lb and is
initially at rest when it is brought into contact with D.
Determine the time required for disk E to attain the same
angular velocity as disk D. The coefficient of kinetic
friction between the two disks is 0.3 . Neglect the
weight of bar BC.

2. Relevant equation
Summation of forces and moments.

The Attempt at a Solution


I am confused with the direction of friction. In my calculations, the friction points to the left because the velocity where the disk meets points to the right; which resulted to a wrong answer. However, the correct free body diagram is shown above.
 
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edsoneicc said:
the friction points to the left
That is the force that "what" exerts on "what else?" And what is the reaction force to that?
 
So when I isolate disk D, the friction points to the left to counter the clockwise motion, and when I isolate disk E, it now points to the right. Is this right?
 
Yes.
 
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Thanks!
 

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