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# An objecte of mass 5 kg falls from rest through a vertical distance of 20 m and reaches velocity 10 m/s. How much work is done by push of air on the object $(g=10 m/s^2)$

$(a)\;120\;J\quad (b)\;-750\;J \quad (c)\; -450\;J \quad (d)\;250\; J$

From work energy theorem work done by weight + work done by air $= \Delta KE$
$W_{\large air}=\Delta KE-W_{(\large weight)}$
$\qquad=\large\frac{1}{2}$$mv^2-mgh \qquad=\large\frac{1}{2}$$ \times 5 \times (10)^2-5 \times 10 \times 20$
$\qquad=-750\;J$
Hence b is the correct answer.
edited Jun 11, 2014

+1 vote