Physics Puzzler!

[paigegail]

HELP! I am in grade 12 physics and my teacher gave us a puzzler for the weekend. I need serious help with it and all of my classmates can't figure it out either! The question is :

A 10.8 kg box is 5.0 m from the edge of a 12.96 m high cliff. Bob pushes the box over the edge of the cliff by applying a constant horizontal force to it until the box goes over the edge. The force of friction on the box is 49.0 N and the box lands 7.5 m from the base of the cliff. How much force did Bob apply to the box while he was pushing?

What do I do and can someone show me how to solve it?

 

[mani]

A 10.8 kg box is 5.0 m from the edge of a 12.96 m high cliff. Bob pushes the box over the edge of the cliff by applying a constant horizontal force to it until the box goes over the edge. The force of friction on the box is 49.0 N and the box lands 7.5 m from the base of the cliff. How much force did Bob apply to the box while he was pushing?

If v is the (horiz.) velocity at push-off and t the time of fall;
vt = 7.5; (1/2) gt^2 = 12.96; Find v.
Total work done by all forces till push-off,
(F-49)5 = Kinetic energy at push-off = (1/2) (10.8) v^2