VIT Engineering VIT Engineering Solved Paper-2007

  • question_answer
    Two electrons are moving in opposite direction with speeds 0.8 c and 0.4 c, where c is the speed of light in vacuum. Then the relative speed is about:

    A)  0.4 c           

    B)  0.8 c

    C)  0.9 c           

    D)  1.2 c

    Correct Answer: C

    Solution :

    The particles are moving with velocities \[0.8\text{ }c\] and \[-0.4\text{ }c\]in the laboratory frame, say S frame. Let S be a reference frame in which the particle with velocity \[-0.4\text{ }c\] is at nest. Then, the velocity of S (laboratory) relative to S is \[v=0.4\text{ }c\]. Therefore, the particle which in S has velocity \[u=+0.8\text{ }c\]has a velocity in S given by \[u=\frac{u+v}{1+\frac{uv}{{{c}^{2}}}}\] \[=\frac{0.8c+0.4c}{1+\frac{(0.8c)\,(0.4c)}{{{c}^{2}}}}\] \[=\frac{1.2c}{1+0.32}=0.9c\]


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