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# A block of $2 kg$ is hanging over a smooth string. The other end of the string is pulled by a constant force $F=40N$.The kinetic energy of the particle increases $40 \;J$ in a given interval of time. then $(g=10 m/s^2)$

a) Tension in string is $45 \;N$ b) displacement of the block in the given interval of time is 2m c) Work done by gravity is $-20 \;J$ d) Work done by tension is $80 \;J$

By Work energy theorem
$W_{net}=\Delta KE$
$(40-20)s=40$
$\therefore s=displacement=2m$
Work done by gravity $=(-20) \times 2=-40 \; J$
Work done by tension $=40 \times 2=80 \;J$
Hence d is the correct answer.

edited Feb 10, 2014 by meena.p