A 100 kg box is set into motion down a rough incline from rest. A constant horizontal force F = 20 N.?

The component of F along the plane and the motion are opposite in direction.
Hence, F = 20 N and s = -5 m

Work done by the applied force, F
= F s cosθ
= 20 × (-5) × cos30.0° J
= -87 J

The answer: d. -87J