KVPY Sample Paper KVPY Stream-SX Model Paper-2

  • question_answer
    A particle moves in the x-y plane with velocity \[\vec{v}=a\hat{i}+bx\hat{j},\]where a and b are constants. Initially, the particle was at the origin. The trajectory of the particle is -

    A)           

    B)

    C)          

    D)

    Correct Answer: C

    Solution :

    [C]
    \[\vec{v}=a\hat{i}+bx\hat{j}\]
    \[{{v}_{x}}=a\] and \[{{v}_{y}}=bx\]
    \[\frac{dx}{dt}=a\]
    \[\therefore x=at+C\]
    Since \[x=0\] at \[t=0,\] \[c=0\]
    \[\therefore x=at\]
    \[\frac{dy}{dx}=bx=abt\] ... (1)
    \[\therefore y=\frac{ab{{t}^{2}}}{2}+C\]
    \[\therefore y=\frac{ab{{t}^{2}}}{2}\]
    as \[y=0\]at \[t=0\] ...(2)
    From equation (1) and (2),
    \[y=\frac{ab{{t}^{2}}}{2}=\frac{b}{2a}{{x}^{2}}\]
    Hence the trajectory is a parabola symmetrical about the y-axis.


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