JEE Main & Advanced JEE Main Paper (Held On 12-Jan-2019 Evening)

  • question_answer
    An alpha-particle of mass m suffers -dimensional elastic collision with a nucleus at rest of unknown mass. It is scattered directly backwards losing, 64% of its initial kinetic energy. The mass of the nucleus is [JEE Main Online Paper Held On 12-Jan-2019 Evening]

    A) 4 m                              

    B) 1.5 m

    C) 3.5 m               

    D)   2 m

    Correct Answer: A

    Solution :

    Let M be the mass of the nucleus. Applying conservation of linear momentum, \[mv=m{{v}_{1}}+M{{v}_{2}}\]                                 ...(i) Also, \[\frac{1}{2}mv_{1}^{2}=\frac{36}{100}\frac{1}{2}m{{v}^{2}}\Rightarrow {{v}_{1}}=\frac{6}{10}v\]  ...(i) Applying conservation of kinetic energy, \[\frac{1}{2}m{{v}^{2}}=\frac{1}{2}mv_{1}^{2}+\frac{1}{2}Mv_{2}^{2}\] \[\Rightarrow \frac{1}{2}Mv_{2}^{2}=\frac{64}{100}\frac{1}{2}m{{v}^{2}}\Rightarrow {{v}_{2}}=\frac{8}{10}v\sqrt{\frac{m}{M}}\] Substituting (ii) and (iii) in eqn. (i) \[mv=-\left( \frac{6}{10}v \right)m+M\left( \frac{8}{10}v\sqrt{\frac{m}{M}} \right)\Rightarrow \frac{16}{10}mv=\frac{8}{10}v\sqrt{mM}\]\[\Rightarrow M=4m\]


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