A lorry of mass $15000{\text{ }}kg$ climbs a hill of length $500{\text{ }}m$ 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 $800{\text{ }}N$.
a) Find the work done by the lorry’s driving force.
On its return journey the lorry reaches the top of the hill with speed $20{\text{ }}m{\text{ }}{s^{ - 1}}$ and continues down the hill with a constant driving force of $2000{\text{ }}N$. The resistance to the lorry’s motion is again constant and equal to $800{\text{ }}N$.
b) Find the speed of the lorry when it reaches the bottom of the hill.
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