Physics Puzzler: Solving a Grade 12 Cliff Pushing Problem

[paigegail]

Physics puzzler! Need serious help!

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?

Please help me if you can. How do I go about even starting the problem?

 

[paigegail]

MORE HELP

I did what the first post said to do and I still can't get it. I end up with Fnet= 117.504 N. What the hell am I supposed to do? I've spent hours on this problem and it's driving me crazy!