JEE Main & Advanced Mathematics Rectangular Cartesian Coordinates Question Bank Transformation of axes and Locus

  • question_answer
    The position of a moving point in the XY-plane at time t is given by \[\left( (u\cos \alpha )t,(u\sin \alpha )t-\frac{1}{2}g{{t}^{2}} \right),\] where \[u,\,\alpha ,\,g\]are constants. The locus of the moving point is

    A) A circle

    B) A parabola

    C) An ellipse

    D) None of these

    Correct Answer: B

    Solution :

    Let \[h=u\,\cos \,\alpha \,.\,t,\,\,k=u\,\sin \alpha \,.\,t-\frac{1}{2}g{{t}^{2}},\] then \[t=\frac{h}{u\,\cos \alpha }\]. Putting the value of t in \[k=u\,\sin \alpha \,.\,t-\frac{1}{2}g{{t}^{2}},\] we get  \[k=h\,\tan \alpha -\frac{1}{2}g\frac{{{h}^{2}}}{{{u}^{2}}{{\cos }^{2}}\alpha }\] \[\therefore \,\,\]Locus of (h, k) is \[y=x\tan \alpha -\frac{1}{2}g\frac{{{x}^{2}}}{{{u}^{2}}\,{{\cos }^{2}}\alpha }\], which is a parabola.


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