$A$ block of mass $m$ moving with a velocity $v_0$ on a smooth horizontal surface strikes and compresses a spring of stiffness $k$ till mass comes to rest as shown in the figure. This phenomenon is observed by two observers:

$A$: standing on the horizontal surface

$B$: standing on the block 

To an observer $A$, the work done by spring force is 

37-559

  • A

    negative but nothing can be said about its magnitude

  • B

    $ - \frac{1}{2}mv_0^2$

  • C

    positive but nothing can be said about its magnitude

  • D

    $ + \frac{1}{2}mv_0^2$

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