A variable mass body with an initial mass of 18 kg is moving with a velocity of 3 m/s initially, after some time the velocity increases to 12 m/s, what is the new mass if the body is moving in the absence of any external force?

4.5 kg

If no net external force acts on the system, the total momentum is conserved.

$m_1 v_1 = m_2 v_2$

$18 \text{ kg x } 3 \text{ m/s} = m \text{ x } 12 \text{ m/s}$

$\Rightarrow m = \frac{54}{12} \text{ kg }$

$\Rightarrow m = 4.5 \text{ kg}$