Question

An object is dropped from a height of $20\text{m}$. If a plane mirror is lying on the floor with the reflecting surface facing the object. The speed of image of the ball with respect to the ball just before striking the mirror will be
$($Take $g=10{\text{m/s}}^2)$

• A. $40\text{m/s}$
• B. $30\text{m/s}$
• C. $10\text{m/s}$
• D. $20\text{m/s}$

Answer 1

The correct option is A $40\text{m/s}$

The speed of the object just before striking the mirror will be,
$v_o=\sqrt{2gh}=\sqrt{400}=20\text{m/s}$
As motion is along the normal of the mirror, on applying, $v_o=-v_i$, we get,
$v_i=-20\text{m/s}$
Thus, the velocity of the image $w.r.t$ object,
$v_{io}=v_i-v_o=-20-20=-40\text{m/s}$
$\therefore |v_{io}|=40\text{m/s}$