Question

A ball is thrown from the top of a 50 m high building. In which of the following cases is the time of flight same?

I. Ball is dropped from rest.

II. Ball is thrown vertically upwards with an initial speed of 5 m/s.
III. Ball is thrown horizontally with an initial speed of 10 m/s.
IV. Ball is thrown horizontally with an initial speed of 10 m/s.

• A. I and II only
• B. II and III only
• C. III and IV only
• D. I, III and IV only
• E. I, II, III and IV

Answer 1

Answer: (D)

I, III and IV only

Given : $S_y=50$ m $a_y=g=10m/s^2$

In cases $I,$ $III$ and $IV$ the component of initial velocity in vertical direction is zero i.e $u_y=0m/s$

Using $S_y=u_{y}t+\frac{1}{2}{a}_y{t}^2$

$\therefore$ $50=0+\frac{1}{2}\times 10\times t^2$ $⟹t=\sqrt{10}$ s (for $I$, $III$ and $IV$)

Hence option D is correct.