Question

If 4 seconds is the time, in which a projectile reaches a point P of its path and 5 seconds is the time from P, till it reaches the horizontal plane through the point of projection. The height of P from the horizontal plane passing through the point of projection is

• A. $78.4m$
• B. $98m$
• C. $122.5m$
• D. $220.5m$

Answer 1

The correct option is B $98m$

Total time of flight $=4+5=9$ sec
$9=\frac{2u\sin \theta }{g}.....\left(1\right)$

$S_y=u\sin \theta t-\frac{1}{2}g{t}^2$

(Substituting $t=4s$)
$=\left(u\sin \theta \right)4-8g$
$=18g-8g$ .....(using (1) $u\sin \theta =\frac{9g}{2}$)
$=10g$
$=10\times 9.8$
$=98m$