Question

A boy weighing 50 kg throws a stone of mass 10 kg horizontally with a velocity of $8m{s}^{-1}$. With what velocity does he move after throwing?

• A. $-1.6m{s}^{-1}$
• B. $-16m{s}^{-1}$
• C. $-160m{s}^{-1}$
• D. $-1.9m{s}^{-1}$

Answer 1

The correct option is A $-1.6m{s}^{-1}$

Let mass of boy: $m_1=50kg$

mass of stone: $m_2=10kg$

Velocity of boy after throwing stone:$v_1$

Velocity of stone:$v_2=8m/s$

Using conservation of momentum :

$m_1{v}_1+m_2{v}_2=0$ (initial momentum of the system = 0)
$50\times v_1+10\times 8=0$

$v_1=\frac{-80}{50}$

$v_1=-1.6m/s$

The boy moves with a speed of $1.6m/s$ in the opposite direction of the stone.