Question

Ball A of mass mA = 3 kg moves to the right (positive direction) with a velocity 4 m/s, while ball B moves to the left (negative direction) with a velocity -4 m/s. After the collision, A's velocity is -21 m/s and B's velocity is 1 m/s. What is the mass of B?

Answer 1

Given:

The mass of A is,

$m_A=3\text{ kg}$

The initial velocity of A is,

$u_A=4\text{ m/s}$

The initial velocity of B is,

$u_B=-4\text{ m/s}$

The final velocity of A is,

$v_A=-21\text{ m/s}$

The final velocity of B is,

$v_B=-1\text{ m/s}$

Applying the momentum conservation principle we can write,

$\begin{gathered} m_Au_A+m_Bu_B=m_Av_A+m_Bv_B \\ 3\times4+m_B(-4)=3\times(-21)+m_B\times1 \\ 12-4m_B=-63+m_B \\ 5m_B=75 \\ m_B=15\text{ kg} \end{gathered}$

Hence, the mass of B is 15 kg.