posted by Eric .
Two blocks of masses 40.0kg and 20.0kg are stacked on a table with the heavier block on top. The coefficient of static friction is 0.600 between the top and bottom blocks and 0.300 between the bottom block and the table. What is the required magnitude of a horizontal force that, if applied to the top block, would move the blocks with the same net acceleration?
You want the force greater than the force of friction at the table surface, but not greater than the friction between the two blocks.