# Direction of friction on two stacked boxes

fontseeker

## Homework Statement

Solve the free diagram of two stacked boxes on a frictionless table accelerated by a force.

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## The Attempt at a Solution

I have box 2 stacked on box 1. Box 1 is on a frictionless table and accelerated by a force. However, there is friction between box 1 and box 2.

Now, I was told that this is the diagram for that problem:

I thought that since m1 and m2 are moving to the right, then the frictional force between box 1 and 2 will be to the left, opposite to the direction of motion. Can someone confirm what is the actual direction of friction for the frictional force between box 1 and box 2?

Also, is the action-reaction pair of forces between box 1 and box 2 the normal force of box 1 and the negative normal force of box 1?

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Orodruin
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There is no such thing as "the frictional force between box 1 and box 2", the statement does not contain enough information as you need to specify the object the force is acting on. The frictional force from box 1 on box 2 and the frictional force from box 2 on box 1 form a third law pair and are therefore of the same magnitude but opposite direction. This is also clear from the sketch.

fontseeker
There is no such thing as "the frictional force between box 1 and box 2", the statement does not contain enough information as you need to specify the object the force is acting on. The frictional force from box 1 on box 2 and the frictional force from box 2 on box 1 form a third law pair and are therefore of the same magnitude but opposite direction. This is also clear from the sketch.
I was told there was a coefficient of static and kinetic friction between boxes 1 and 2. So I don't understand how there is no frictional force between them?

Orodruin
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So I don't understand how there is no frictional force between them?
That is not what I said. I said your statement was not sufficiently accurate to identify which of the frictional forces you were asking about. The one from 1 on 2 or the one from 2 on 1. They form a third law pair and are therefore in opposite directions.

fontseeker
That is not what I said. I said your statement was not sufficiently accurate to identify which of the frictional forces you were asking about. The one from 1 on 2 or the one from 2 on 1. They form a third law pair and are therefore in opposite directions.
So this is a similar problem on where I have that scenario:

How would I identify the direction of friction in this problem?

Orodruin
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How would I identify the direction of friction in this problem?
Again, there is no such thing as ”direction of friction”. In order to talk about the direction of a force you must specify what force you are talking about. Just saying ”friction” is not enough to do that. You must specify what object the frictional force you are interested in acts on.

fontseeker
Again, there is no such thing as ”direction of friction”. In order to talk about the direction of a force you must specify what force you are talking about. Just saying ”friction” is not enough to do that. You must specify what object the frictional force you are interested in acts on.
I am interested in both object 1 and 2. I want to find the direction of friction for those two objects separately, since I need to create a free-body diagram of both objects completely independently.

Orodruin
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I am interested in both object 1 and 2. I want to find the direction of friction for those two objects separately, since I need to create a free-body diagram of both objects completely independently.
If you know one you will know the other. They form a third law pair. So pick one of the objects, say the top one. What do you think the direction of the frictional force acting on that object is and why?

fontseeker
If you know one you will know the other. They form a third law pair. So pick one of the objects, say the top one. What do you think the direction of the frictional force acting on that object is and why?
From personal experience, I know that the top box will fall behind, meaning the friction will go to the right on the top box. However, I don't understand why the box falls to the right.

haruspex