A ball is projected upwards from the top of a tower with a velocity of 50 ms-1 making an angle of 30° with the horizontal. The height of the tower is 70 m. After how many seconds from the instant of throwing will the ball reach the ground ?
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a
2 s
b
5 s
c
7 s
d
9 s
answer is C.
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Detailed Solution
The vertically upward component of the velocity of projection = 50 sin 30° m / s =25 ms-1If t is the time taken to reach the ground, we haves=−u0t+12gt2or 70=−25t+1210t2or 5t2 - 25t -70 = 0 so t = -2s or t = 7sHere, t = -2s is not valid.