A box is pushed down an incline from point A to point B by a force F (and also under the influence of its weight). The distance from A to B, along the ramp, is 5.0 m. The force F is parallel to the incline, as shown, and has a magnitude of 2.0 N. The force of kinetic friction between the box and the ramp is 10.0 N. The angle of incline of the ramp is 40.0o. The change in kinetic energy of the box in moving from A to B is +35 J. What is the mass of the box?

hmax = L*sin A = 5*sin40 = 3.21 m.

At top of incline:
KE + PE = mg*hmax-Fk*L
0 + PE = mg*3.21-10*5
PE = mg*3.21-50.

At bottom of incline:
KE + PE = mg*3.21-50
KE + 0 = mg*3.21-50
KE = mg*3.21-50 = 35 J.
m*9.8*3.21-50 = 35
31.5m-50 = 35
31.5m = 85
m = 2.7 kg. = Mass of box.