1. The problem statement, all variables and given/known data Look at the figure below. Derive the formula for the magnitude of the force F exerted on the large block (mC) in the figure such that the mass mA does not move relative to mC. Ignore all friction. Assume mB does not make contact with mC. 2. Relevant equations F = ma 3. The attempt at a solution So mB is pulling the block mA to the right. Therefore if the block mC were to not move, mA would move to the right. So the acceleration of mA needs to be equal to the acceleration of mC to make sure the blocks stay at the same relative position. mA is accelerating at the rate of g since mB is accelerating at the rate of g. To accelerate mC at the rate of g, you'd need to move both the mass of mC and mB therefore I thought the force would be (mB + mC)g. This is wrong and I'm not sure why.