Question

2 plastic balls have masses of $100g$ and $200g$. The $100g$ ball is moving at $3m{s}^{-1}$ and hits the $200g$ stationary ball. They collide with each other and after the collision, the $100g$ ball moves with a velocity of $1.5m{s}^{-1}$ in the same direction. Calculate the velocity of the other ball after collision.

• A. $0.75m{s}^{-1}$
• B. $0.75km{s}^{-1}$
• C. $0.75cm{s}^{-1}$
• D. $0.57m{s}^{-1}$

Answer 1

The correct option is A $0.75m{s}^{-1}$

Let $m_1=100g=0.1kg$ be the mass of first ball, $m_2=200g=0.2kg$ be the mass of second ball.
$u_1$ initial velocity of first ball before collision $=3m{s}^{-1}$
$u_2$ initial velocity of second ball before collision $=0m{s}^{-1}$
$v_1$ final velocity of first ball after collision $=1.5m{s}^{-1}$
$v_2$ final velocity of second ball after collision
Applying law of conservation of momentum we can write,
$m_1{u}_1+m_2{u}_2=m_1{v}_1+m_2{v}_2$
Substituting values
$0.1\times 3+0.2\times 0=0.1\times 1.5+0.2\times v_2$
$0.3=0.15+0.2\times v_2$
Or $v_2=0.75m{s}^{-1}$