Consider an elliptically shaped rail PQ in the vertical plane with OP = 3 m and OQ = 4m. A block of mass 1 kg is pulled along the rail from P to Q with a force of 18 N, which is always parallel to line PQ (see the figure given). Assuming no frictional losses, the kinetic energy of the block when it reaches Q is (n × 10) Joules. The value of n is (Take acceleration due to gravity = 10 ms–2)

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G1 Physics · Accepted Answer

The force of 18 N is always parallel to the fixed line PQ, so its work is F·PQ=18×5=90 J because PQ=√(3²+4²)=5 m. The block rises 4 m, so gravity does -40 J. Starting from rest, K(Q)=90-40=50 J=(n×10) J, hence n=5.