posted by Sarah .
Search: The figure shows three crates being pushed over a concrete floor by a horizontal force of F magnitude 440 N. The masses of the crates are m1 = 30.0 kg, m2 = 10.0 kg, and m3 = 20.0 kg. The coefficient of kinetic friction between the floor and each of the crates is 0.700. (a) What is the magnitude F32 of the force on crate 3 from crate 2? (b) If the crates then slide onto a polished floor, where the coefficient of kinetic friction is less than 0.700, is magnitude F32 more than, less than, or the same as it was when the coefficient was 0.700?
Since the Figure is not available to us, we need to know which of the crates receives the external 440 N force.
Problems of this sort can be solved by applying free-body-diagram equations of motion to each of the three crates, remembering that the forces between any two crates that are in contact are equal and opposite. This allows one to solve for all three unkowns: F32 = -F23, F12 = -F21, and the acceleration, a.