Question

A pebble is thrown vertically upwards from bridge with an initial velocity of $4.9m/s$. It strikes the water after $2s$. If acceleration due to gravity is $9.8m/s^2$. The height of the bridge & velocity with which the pebble strikes the water will respectively be-

• A. $4.9m,1.47m/s$
• B. $9.8m,14.7m/s$
• C. $49m,1.47m/s$
• D. $1.47m,4.9m/s$

Answer 1

The correct option is B $9.8m,14.7m/s$

$\text{Given data}$

$u=4.9m/s$

$a=-9.8m/s$

$t=2s$

Taking the point of projection as origin and downward direction as positive,

By second equation of motion, i.e.,

$s=ut+\frac{1}{2}a{t}^2$

$s=h,\text{after}t=2s$

$h=4.9\times 2+\frac{1}{2}(-9.8)(2{)}^2$

$h=9.8-2\left(9.8\right)$

$h=-9.8m$

$|h|=9.8m$

$\text{velocity after 2s}$

By first equation of motion, i.e.,

$v=u+at$

$v=4.9+(-9.8)\left(2\right)$

$v=-14.7m/s\downarrow$