How would you go about finding the force of this object?

[Merlan114]

A wrench slides off a tilted shelf, although if a force of 1.6N is applied opposite the wrench's motion the wrench will slide down the shelf with a constant velocity. If the shelf is 1.2m long, what is the work done by the applied force on the wrench?
This is what I did:
w=(1.6N)(1.2m)= and got 1.92 J
On the worksheet it said that I have to multiply it also by cos 180, but I'm not understanding where they got cos 180 from. Can someone please explain?

 

[PhizKid]

The force is being applied in the opposite direction of the displacement for work. Therefore, you have to use 180 degrees instead of 0 degrees.