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Say you have block1 on top of block 2 and the whole system is accelerating toward the right at a certain acceleration. Due to inertia block 1 wants to move backward to the left so there has to be a force of static friction on it that acts to the right in order for it to stay in place.

Let's say the maximum static force that block 2 can put on block 1 is equal to $\mu N=M_1a$. That is also the maximum acceleration of the system, if acceleration is to be greater than this maximum:

  1. would the block would start to moving to the left?

  2. would the kinetic friction force still points in the same direction as the static friction force?

  3. if 1 and 2 is true, is the kinetic friction force greater than the force of static friction?

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  • $\begingroup$ What force would make block 1 move to the left? There is only inertia (stay in place) and friction (move to the right). Am I missing something? Maybe you are thinking about this in the frame of reference of block 2? Please clarify. $\endgroup$
    – Floris
    Commented Sep 16, 2014 at 22:54
  • $\begingroup$ Ok if the system accelerates to the right, doesn't matter what the force is, the top block has an acceleration to the right that is caused by friction correct? $\endgroup$
    – Loc Tran
    Commented Sep 16, 2014 at 23:52
  • $\begingroup$ @Floris Yes my money is that Loc Tran means block 1 would move to the left relative to block 2. $\endgroup$
    – BMS
    Commented Sep 16, 2014 at 23:52
  • $\begingroup$ If it moves to the left relative to block 2, that's the force kinetic friction and the direction of that force is to the right because the whole system is moving to the right $\endgroup$
    – Loc Tran
    Commented Sep 16, 2014 at 23:54
  • $\begingroup$ if it doesn't move to the left, then there is a force of static friction on the block, this force is also to the right. I'm asking if the force of kinetic friction would be greater than the force of static friction $\endgroup$
    – Loc Tran
    Commented Sep 16, 2014 at 23:55

1 Answer 1

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  1. No, block 1 would move to the right but with smaller acceleration than block 2. You could say it would move to the left relative to the block 2.

  2. Yes it would, since block 1 still 'wants' to move to the left relative to the block 2 (the 'relative' part is very important)

  3. No it wouldn't, it would be smaller. Block 1 would now move with acceleration that is less then mentioned maximum acceleration, because frictional force (which was giving block 1 the acceleration) has now decreased. Kinetic friction is always smaller than stationary. (if anyone knows any examples of the opposite, please comment)

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  • $\begingroup$ I think i have to disagree with you response to number 1. I think that block 1 would have the same acceleration as block 2 because they are in a system and the force is applied on block 2. If the force was to be applied on block 1, then yes you are right $\endgroup$
    – Loc Tran
    Commented Sep 17, 2014 at 2:55
  • $\begingroup$ They are in a system as long as there is a force that ties them together. As soon as kinetic friction starts to act it means the blocks are moving relative to one another (otherwise it wouldn't be kinetic friction), and two objects moving relative to each other are not in the same system. While static friction acts they are a system since they move together, and static friction is the force that binds them. $\endgroup$
    – DoctorJAM
    Commented Sep 17, 2014 at 9:39

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