At high altitude, a body explodes at rest into two equal fragments with one fragment receiving horizontal velocity of $10 \,m/s$. Time taken by the two radius vectors connecting point of explosion to fragments to make $90^o $ is ............ $\mathrm{s}$

  • A

    $10$

  • B

    $4$

  • C

    $2$

  • D

    $1$

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  • [AIPMT 2010]

Two particles having position vectors $\overrightarrow {{r_1}} = (3\hat i + 5\hat j)$ metres and $\overrightarrow {{r_2}} = ( - 5\hat i - 3\hat j)$ metres are moving with velocities ${\overrightarrow v _1} = (4\hat i + 3\hat j)\,m/s$ and ${\overrightarrow v _2} = (\alpha \,\hat i + 7\hat j)$ $m/s.$ If they collide after $2$  seconds, the value of $'\alpha '$ is