Q 11-05-187JEE MainJEE Main 2019 (9 Apr, Shift 1)Easy
A body of mass $2\ \text{kg}$ makes an elastic 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?
Answer: (B) $1.2\ \text{kg}$
For a one-dimensional elastic collision with the second body at rest:
$$v_1' = \frac{m_1 - m_2}{m_1 + m_2}v_1 = \frac{v_1}{4}$$
$$4(2 - m_2) = 2 + m_2 \Rightarrow m_2 = 1.2\ \text{kg}$$
Solution by Sreeraj P, M.Sc Physics