A block of mass mt=4.0 kg is put on top of a block of mass mb=5.0 kg. To cause the top block to slip on the bottom one while the bottom one is held fixed, a horizontal force of at least 12N must be applied to the top block. The assembly of blocks is now placed on a horizontal, frictionless table (Fig. 6-51). Find the magnitudes of (a) the maximum horizontal force F that can be applied to the lower block so that the blocks will move together and (b) the resulting acceleration of the blocks. Link to page with question: http://www.ifm.liu.se/edu/coursescms/TFYA16/lessons/Le-3-extra.pdf [Broken] It's on the 5th page. I don't understand their equation for maximum horizontal force (M a = M μ_s g) and would appreciate if someone would explain it to me.