Question

50. A ball thrown vertically upwards with speed 'u' from the top of a tower reaches the ground in
9 seconds. Another ball thrown vertically downwards from the same position with speed 'w', takes
4 seconds to reach ground. Calculate the value of'u'. (Take g = 10 ms)
​

Answer 1

Explanation:

Ball is thrown from the top of tower, ⇒height=−h,

For the first case, velocity = u , Time t=9s,

Using s=ut+

2

1

at

2

⇒−h=u×9−

2

1

×10×9

2

⇒−h=9u−405 .....1

For the second case, velocity = −u , Time t=4s,

Using s=ut+

2

1

at

2

⇒−h=−u×4−

2

1

×10×4

2

⇒−h=−4u−80 .....2

Equating equations 1 and 2,

⇒9u−405=−4u−80 ⇒u=25ms

−1