Q) A lorry of mass 15 000 kg moves with constant speed 14ms−1 from the top to the bottom of a straight hill of length 900 m. The top of the hill is 18m above the level of the bottom of the hill. The total work done by the resistive forces acting on the lorry, including the braking force, is 4.8 × 106 J.

Find

(i) the loss in gravitational potential energy of the lorry

(ii) the work done by the driving force.

The answer to part (i) is mgh = 2700000J

But I am stuck at (ii)

I used the formula

WDF - Work Done By Resistance = 2700000

WDF = 2700000+4.8x10^6

WDF = 7500000J

But the correct answer is 2.1 x 10^6J

How? Am I using a wrong method?

Find

(i) the loss in gravitational potential energy of the lorry

(ii) the work done by the driving force.

**My Attempt**The answer to part (i) is mgh = 2700000J

But I am stuck at (ii)

I used the formula

WDF - Work Done By Resistance = 2700000

WDF = 2700000+4.8x10^6

WDF = 7500000J

But the correct answer is 2.1 x 10^6J

How? Am I using a wrong method?

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