Question

By which velocity a ball be projected vertically downward so that the distance covered by it in 5th second is twice the distance it covers in its 6th second ($g = 10m/s^{2}$)

• A. $58.8m/s$
• B. $49m/s$
• C. $65m/s$
• D. $19.6m/s$

Answer 1

Answer: (C)

$65m/s$

Answer 2

By formula $h_{n} = u + \frac{1}{2}g(2n - 1)$

⇒$\mathbf{u}\mathbf{-}\frac{\mathbf{10}}{\mathbf{2}}\mathbf{\lbrack 2}\mathbf{\times}\mathbf{5}\mathbf{-}\mathbf{1\rbrack = 2\{ u}\mathbf{-}\frac{\mathbf{10}}{\mathbf{2}}\mathbf{\lbrack 2}\mathbf{\times}\mathbf{6}\mathbf{-}\mathbf{1\rbrack\}}$

⇒$u - 45 = 2 \times (u - 55)$ ⇒ $u = 65m/s.$