Question

A projectile is fired horizontally with a speed of $98m{s}^{-1}$ from the top of a hill $490m$ high. Find the time taken to reach the ground.

• A. $5s$
• B. $10s$
• C. $15s$
• D. $20s$

Answer 1

The correct option is B $10s$

Answer is B.
The projectile is fired from the top $O$ of a hill with speed u = $98m{s}^{-1}$ along the horizontal as shown as $OX$. It reaches the target P at vertical depth OA, in the coordinate system as shown, $OA=y=490m$

As, $y=\frac{1}{2}g{t}^2$

$490=\frac{1}{2}\times 9.8t^2$

or $.t=\sqrt{100}=10s$

Hence, the time taken to reach the ground is $10s$.