1. The problem statement, all variables and given/known data Okay guys, I'm new here, and this problem just annoyed me because I believe it's very simple, but it looks like it could be more complicated. Two large crates, with masses 640 kg and 490 kg, are connected by a stiff, massless spring (k=8.1 kN/m) and propelled along an essentially frictionless, level factory floor by a force applied horizontally to the more massive crate. If the spring compresses 5.1 cm from its equilibrium length, what is the applied force? 2. Relevant equations I think it's just this one: Fspring = -k*x 3. The attempt at a solution Okay, the only thing that makes it sound at all complicated is that the crates are different masses, and that they talk about mass at all. But I want to say it's just 8.1*(.051) to get kN. Is that it? Or should I take in to account the masses of the boxes. In which case I'm not quite sure where to go.