# How does the static friction $\vec{F_{1,2}}$ and $\vec{F_{2,1}}$ generate here?

How does the static friction $$\vec{F_{1,2}}$$ and $$\vec{F_{2,1}}$$ generate here? I am confusing with $$\vec{F_{1,2}}$$ and $$\vec{F_{2,1}}$$

My understanding from the video:- If we push the block (2) with a force $$\vec{F}$$, If it is moving. Block (2) is sitting above block (1). So, by Newton's First Law of Inertia. Block (2) have a tendency to stay in the initial position. If there is friction on both surfaces. When we apply $$\vec{F}$$ to block (1). it will move right. So, Block (2) move left. So, there is static friction opposes the block to move in the left direction. Let it be $$\vec{F_{1,2}}$$. Am I correct?

If Block (1) is pushed right. The bottom surface of the block (2) opposes the motion. So, It will be $$F_{1,2}$$ act in the left direction. Is my logic correct in both cases?

How does the static friction 𝐹1,2 and 𝐹2,1 generate here? I am confusing with 𝐹1,2 and 𝐹2,1

Block 1 exerts a static friction force of $$F_{12}$$ on block 2 and block 2 exerts an equal and opposite static friction force of $$F_{21}$$ on block 1 as long as there is no relative motion. It is only block 1 that can cause block 2 to move to the right by virtue of the friction force that block 1 exerts on block 2.

My understanding from the video:- If we push the block (2) with a force 𝐹⃗ , If it is moving. Block (2) is sitting above block (1). So, by Newton's First Law of Inertia. Block (2) have a tendency to stay in the initial position.

Newton's 1st law states that a body at rest will remain at rest and a body in motion will remain in motion, UNLESS acted upon by a net external force. Block 2 will only have a tendency to stay in its initial position if there are no external forces acting upon. But in this case, there is an external force acting on it, namely the friction force of block 1.

If there is friction on both surfaces. When we apply 𝐹⃗ to block (1). it will move right. So, Block (2) move left. So, there is static friction opposes the block to move in the left direction. Let it be 𝐹1,2→. Am I correct?

If the force $$F$$ acts on block 1, then block 2 will move along with block 1 to the right as long as the force $$F_{12}$$ does not exceed the maximum static friction force of $$μ_{s}m_{2}g$$ where $$μ_{s}$$ is the coefficient of static friction between blocks 1 and 2. At that point block 2 will start slipping on block 1 and experience a kinetic friction force which is generally less than the static friction force. Block 2 will never move to the left with respect to the frame of reference of the supporting surface. It will move to the left with respect to the frame of reference of block 1 if the maximum static friction force is exceeded.

Hope this helps.

• as long as there is no relative motion. It only blocks 1 that can cause block 2 to move to the right by virtue of the friction force that block 1 exerts on block 2. What do you mean by this? would you please explain bit more? – Unknown x Jul 22 '19 at 1:39
• Why he is not taking any reaction force on the system. When we apply $\vec{F}$? – Unknown x Jul 22 '19 at 1:41
• @unknownx Regarding the first comment, look at the FBD for block 2. The only external horizontal force acting on block 2 is the static friction force $F_{12}$ to the right. That’s the force that will cause the block to accelerate. Regarding the second comment how do you wish to define the “system”? – Bob D Jul 22 '19 at 6:45
• External force $\vec F$ is acting to block 1. right? won't the block exert equal and opposite reaction to the person/force supplier? – Unknown x Jul 22 '19 at 9:40
• None of the FBD they are not mentioning this. Why? – Unknown x Jul 22 '19 at 9:41