Forces between blocks
Three blocks are side by side and a force is applied pushing the left-most block to the right. I found the acceleration of the all the blocks by treating them like one big mass, but now I have to find the interaction force between the first and second block. Do I just set that force equal to the mass of those two blocks times the acceleration I found earlier?
Answer
Your strategy might give you an intuitive answer, but the problem is that you don't know about the interaction forces. Instead, let's try another approach by drawing a diagram and drawing the forces on each individual block.
The left-most block has the applied force (green) pushing it to the right, but the normal force with the second block pushing against it. The middle block has two normal forces on it, one pushing it (black) and one opposing it (blue). The rightmost block only has one force acting on it, the normal force due to the middle block (red).
You know that all three blocks will have the same acceleration, so you can use Newton's 2nd law for each block to make an equation in terms of the forces on it and the acceleration. Is there anything else we can determine about this problem? Think about what Newton's 3rd law tells you: each pair of normal forces must be equal and opposite.
Starting with the rightmost block: do you have an expression for the normal force (red arrow)? Can you solve the middle block's equation knowing that the left-facing normal force (blue) is equal and opposite? Try working your way backwards to the original applied force which will let you solve for all of the interaction forces.
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Your strategy might give you an intuitive answer, but the problem is that you don't know about the interaction forces. Instead, let's try another approach by drawing a diagram and drawing the forces on each individual block.
The left-most block has the applied force (green) pushing it to the right, but the normal force with the second block pushing against it. The middle block has two normal forces on it, one pushing it (black) and one opposing it (blue). The rightmost block only has one force acting on it, the normal force due to the middle block (red).
You know that all three blocks will have the same acceleration, so you can use Newton's 2nd law for each block to make an equation in terms of the forces on it and the acceleration. Is there anything else we can determine about this problem? Think about what Newton's 3rd law tells you: each pair of normal forces must be equal and opposite.
Starting with the rightmost block: do you have an expression for the normal force (red arrow)? Can you solve the middle block's equation knowing that the left-facing normal force (blue) is equal and opposite? Try working your way backwards to the original applied force which will let you solve for all of the interaction forces.