Question

A projectile is projected with an initial velocity $\left(4\hat{i}+5\hat{j}\right)m/s$. Here $\hat{j}$ is the unit vector directed vertically upwards and unit vector $\hat{i}$ is in the horizontal direction .Velocity of the projectile (in $m/s$) just before it hits the ground is

• A. $4\hat{i}+5\hat{j}$
• B. $-4\hat{i}+5\hat{j}$
• C. $4\hat{i}-5\hat{j}$
• D. $-4\hat{i}-5\hat{j}$

Answer 1

Answer: (C)

$4\hat{i}-5\hat{j}$

Answer 2

The projectile is moving under gravity in which vertically downward gravitational force acts on it. Therefore, horizontal velocity of the projectile remains same as $(4 \hat{ i }) m / s$. Trajectory of the projectile is a parabola which is symmetric as diagram given below shows. Therefore, velocity of the projectile in vertical direction when it hits the ground will be $(-5 \hat{ i }) m / s$. So, velocity of the projectile when it hits the ground, $v=4 \hat{ i }-5 \hat{ j }$