An office chair has a compressed spring that allows the chair to tilt backward when a sufficiently large force F is applied. When F is small, the stop at point A prevents the chair from tilting...

An office chair has a compressed spring that allows the chair to tilt backward when a sufficiently large force F is applied. When F is small, the stop at point A prevents the chair from tilting forward. If the spring has 10 lb/in. stiffness and 15 in. unstretched length, determine the value of F that will cause the chair to begin tilting backward.