Physics · JEE | WORK, ENERGY AND POWER

What is the fractional kinetic energy lost \( f_1 = \frac{K_{1f}}{K_{1i}} \) by a neutron of mass \( m_1 \) in an elastic collision with a moderating nucleus of mass \( m_2 \)?

1. A
   \( f_1 = \frac{m_1}{m_1 + m_2} \)
2. B
   \( f_1 = \frac{4m_1 m_2}{(m_1 + m_2)^2} \)
3. C
   \( f_1 = (\frac{m_2 - m_1}{m_1 + m_2}) \)
4. D
   \( f_1 = (\frac{m_1 - m_2}{m_1 + m_2})^2 \)