Q. A body is projected vertically upwards with a velocity ‘u’. It crosses a point in its journey at a height ‘h’ twice, just after 1 and 7 seconds. The value of u in ms-1 is (g = 10ms-2)
(a) 50
(b) 40
(c) 30
(d) 20
Ans: (b)
Sol:
$ \displaystyle h = ut – \frac{1}{2}gt^2 $
$ \displaystyle gt^2 -2ut +2h = 0
$
$ \displaystyle t_1 +t_2 = \frac{2u}{g} and \quad t_1.t_2 = \frac{2h}{g} $
1+7 = 2u/g
u = 4g = 40 m/s