# Directions and magnitudes of static and kinetic friction between stacked blocks [closed]

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?

• 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. Commented Sep 16, 2014 at 22:54
• 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? Commented Sep 16, 2014 at 23:52
• @Floris Yes my money is that Loc Tran means block 1 would move to the left relative to block 2.
– BMS
Commented Sep 16, 2014 at 23:52
• 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 Commented Sep 16, 2014 at 23:54
• 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 Commented Sep 16, 2014 at 23:55