6 A lorry of mass 15000 kg climbs a hill of length 500 m at a constant speed. The hill is inclined at \(2.5 ^ { \circ }\) to the horizontal. The resistance to the lorry's motion is constant and equal to 800 N .
- Find the work done by the lorry's driving force.
On its return journey the lorry reaches the top of the hill with speed \(20 \mathrm {~m} \mathrm {~s} ^ { - 1 }\) and continues down the hill with a constant driving force of 2000 N . The resistance to the lorry's motion is again constant and equal to 800 N .
- Find the speed of the lorry when it reaches the bottom of the hill.