A. A box slides down a ramp from the back of a delivery truck. (See Fig. 7.12a from text, also shown at right.) Assume here that the ramp is frictionless. System: the box and the Earth. Does the energy of this system increase, decrease, or remain unchanged? Explain. K ii) h iii) mass m i) Earth exerts a gravitational force (WEB). The ramp exerts a normal force on the box (nR>B). ii) Earth does positive work because there is an acute angle between the force and displacement. The ramp does zero work because the normal force is perpendicular to the displacement. B. Consider again the situation from part A, but now treat the ramp as rough (not frictionless) and suppose that the box slides up the ramp with decreasing speed. ii) B iii) We use WNC = AEsystem here because the Earth is part of the system. (If the system has included just the box, then we would need to use "Wtotal = AKbox" instead. Why?) The only non-conservative force present is the normal force by the ramp, and the work done by this normal force is z