When you have a line of 4 boxes without gaps between adjacent boxes, and all of them are at rest, what exact forces are involved when a rightward force is applied to the leftmost box to cause it to move straight into the other boxes? To make this easier, lets assume no gravity, friction, or air resistance is involved. Here is a picture of the situation: I have been trying various ways to solve this. One thing I did was figure out how a stack of objects sitting on the ground would interact with eachother (https://www.physicsforums.com/showthread.php?t=417332"). I was hoping this would clear things up, but I am still confused as to how this works. Here is what I came up with: To me, this looks like the force being applied to A causes D to accelerate leaving A, B, and C at rest. However, as soon as D moves, C is no longer applying F4 to it so it cannot move. I have been trying to solve this "chicken and egg" problem for a while, but I can't quite get my head around it... Thinking of ABCD as a single object of mass A_mass + B_mass + C_mass + D_mass works of course, but since they don't actually form a single solid object, I need to somehow figure out how the individual objects are affecting eachother. I was able to figure out that the net force of each object is rightward facing and has a magnitude equal to a fraction of the force pushing object A to the right. The fraction is based on the mass of the object in relation to the total mass (as I type this I'm guessing it is (mass/total_mass)*force), but I can't figure out how that net force comes about. What exact forces were involved to create that net force?