• # question_answer A body of mass 2 kg makes an eleastic collision with a second body at rest and continues to move in the original direction but with one fourth of its original speed. What is the mass of the second body? [JEE Main 9-4-2019 Morning] A) 1.8 kg              B) 1.2 kgC) 1.5 kg              D) 1.0 kg

By conservation of linear momentum :- $2{{\text{v}}_{0}}=2\left( \frac{{{\text{v}}_{0}}}{4} \right)+m\text{v}\Rightarrow \text{2}{{\text{v}}_{0}}=\frac{{{\text{v}}_{0}}}{2}+m\text{v}$ $\Rightarrow \frac{\text{3}{{\text{v}}_{0}}}{2}=m\text{v}$??.(1) Since collision is elastic$\to$ ${{\text{V}}_{\text{separation}}}\text{=}\,{{\text{v}}_{\text{approch}}}$ $\Rightarrow \text{v-}\frac{{{\text{v}}_{0}}}{4}={{\text{v}}_{0}}\Rightarrow \frac{\text{5}{{\text{v}}_{0}}}{4}=\text{v}$?..(2) equating (2) and (1) $\frac{\text{3}{{\text{v}}_{0}}}{2}=m\left( \frac{\text{5}{{\text{v}}_{0}}}{4} \right)\Rightarrow \,=m=\frac{6}{5}=1.2kg$ Option [b]