1. The problem statement, all variables and given/known data If you have a system with three boxes, two side by side and one on top of the first box and there is a horizontal force acting on the boxes, do you have to account for the third box in calculating the contact force between two of them? Say if box 1, box 2, and box 3 (three is on top of box 1) had masses of 10 kg, 5 kg, and 6 kg, respectively. And the horizontal force was equal to 20 N and you were asked to calculate the contact force between boxes 1 and 2...would you have to divide 20N by the sum of all THREE masses or just the masses of ONE and TWO in order to get the acceleration to calculate contact force? 2. Relevant equations You would need to use F = ma (a = F/m) to get the acceleration in the horizontal direction and then to get the contact force. 3. The attempt at a solution I thought that you would have to take the box on top into account so a = 20 N / (10 kg + 5 kg + 6 kg) = 0.95 m/s^2 And then to get the contact force between boxes 1 and 2, you would multiply 0.95 m/s^2 by 5 kg to get 4.76 N.