A 3.628 kg freight car moving along a horizontal rail road spur track at $7.2\; km/hour$ strikes a bumper whose coil springs experiences a maximum compression of $30 \;cm$ in stopping the car. The elastic potential energy of the springs at the instant when they are compressed $15\; cm$ is [2013]

(a) $12.1 \times 10^4\;J$ (b) $121  \times 10^4\;J$ (c) $1.21 \times 10^4\;J$ (d) $1.21  \times 10^4\;J$

  • A

    zero

  • B

    $mgvt$ $\cos ^2 \theta$

  • C

    $mgvt$ $\sin ^2 \theta$

  • D

    $mgvt$ $\sin 2 \theta$

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