Work at an Angle

A 200 N rope pulls a crate 5.0 m horizontally. The rope is 30° above the horizontal. Find the work done by the tension.

A 12 N friction force acts on a sliding box for 3.0 m. Calculate the work done by friction and state what its sign means.

Worked answer: $W=12\times3.0\times\cos180^\circ=-36\text{ J}$. The negative sign means friction transfers 36 J away from the box's mechanical energy store.