# Kinetic friction direction on two stacked blocks

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## Main Question or Discussion Point

If i have two blocks on top of each other and there is friction between the two blocks and it's a kinetic friction ( given μk). if i was given the force applied on the lower block to the right direction for example. ignoring the friction between the lower block and the ground, i tried to draw the Free body diagram for both blocks and i got somehow confused with choosing the right direction of the Fk (Kinetic friction) for the upper block.

this is how I thought about it:
first, I know that the friction basically is opposite to the direction of movement. so since the upper block will move to the left (with respect to lower block) that the kinetic friction will be directed to the right. but i thought this one is not right because what force other than fk will make the block move to left ( i should have a force to the left so it moves to left) so I came to say it should be to the left. but then how come the friction be parallel to the direction of motion when it should be opposite?

so I concluded that what happens actually and realistically is that the upper block will move to the right and the force responsible of this is the kinetic friction itself, but if we see it with respect to lower block so it is moving to left and friction is opposite so it's to right!

does that mean that friction is defined opposite to the motion with respect to the lower block only! i am confused and can't decide the right way to think about it

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Doc Al
Mentor
first, I know that the friction basically is opposite to the direction of movement.
That's what's messing you up. Better to think of friction as opposing slipping between surfaces.

If the bottom block accelerates to the right, friction will drag the top block along. If the blocks slip with respect to each other, then kinetic friction acts. Note that with respect to the bottom block, the top block slips to the left thus friction will act toward the right. The bottom block exerts a friction force on the top block that acts to the right and, per Newton's 3rd law, the top block exerts an equal and opposite friction force on the bottom block that acts to the left.

PeroK
Homework Helper
Gold Member
Also, the top block accelerates to the right. It only moves to the left relative to the bottom block. I.e. it has a lower acceleration to the right than the lower block. But the acceleration is to the right.

That's what's messing you up. Better to think of friction as opposing slipping between surfaces.

If the bottom block accelerates to the right, friction will drag the top block along. If the blocks slip with respect to each other, then kinetic friction acts. Note that with respect to the bottom block, the top block slips to the left thus friction will act toward the right. The bottom block exerts a friction force on the top block that acts to the right and, per Newton's 3rd law, the top block exerts an equal and opposite friction force on the bottom block that acts to the left.
Great i got it! thanks!
just to be sure can you please tell me if this Free body diagram is correct or not incase the upper block is the one being pulled by the force not the lower block. so i make sure i am understanding it right ! please check the attachment and forgive me for my bad drawing!

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Doc Al
Mentor

As long as you realize, as I'm sure you do, that the normal forces are different.

On block A, block B exerts an upward normal force that happens to equal $m_Ag$.

On block B, two normal forces act: A downward normal force from block A that equals $m_Ag$, and an upward normal force from the table that equals $m_Ag + m_Bg$.

On block B, two normal forces act: A downward normal force from block A that equals mAgmAgm_Ag, and an upward normal force from the table that equals mAg+mBgmAg+mBgm_Ag + m_Bg.
Yeah, i Know that but I found it somehow hard to write with the painter :) ( you can see how I am struggling to write the letters ^_^) Anyways thanks a lot, I must have pointed the difference in fn to avoid any confusions for other readers! Appreciate your quick Help.