Q: A bullet of mass 10gm is fired horizontally with a velocity 1000m/s from a riffle situated at a height 50m above the ground. If the bullet reaches the ground with a velocity 500m/s , the work done against air resistance in the trajectory of the bullet is (in joule) (g = 10ms-2)
(a) 5005
(b) 3755
(c) 3750
(d) 17.5
Ans: (b)
Sol:
work done against air resistance Wf = Loss in K.E + Loss in P.E
$ \displaystyle W_f = \frac{1}{2}m (1000)^2 – \frac{1}{2}m (500)^2 + m h (50) $
$ \displaystyle W_f = \frac{1}{2}10 \times 10^{-3} (1000+500)(1000-500) + 10 \times 10^{-3}\times 10 \times 50 $
Wf = 3755 J