First slide

Projection Under uniform Acceleration

Question

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 ?

Moderate

A

2 s
B

5 s
C

7 s
D

9 s

Solution

The vertically upward component of the velocity of projection = 50 sin 30° m / s =25 ms^-1
If t is the time taken to reach the ground, we have
$s = - u_{0} t + \frac{1}{2} {gt}^{2}$
or $70 = - 25 t + \frac{1}{2} 10 t^{2}$
or 5t² - 25t -70 = 0 so t = -2s or t = 7s
Here, t = -2s is not valid.

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