Two large crates, with masses 640 kg and 490 kg , are connected by a stiff, massless spring (k=8.1kN/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?
a) 320 N
b) 410 N
c) 530 N
d) 620 N