$A$ block of mass $m$ moving with a velocity $v_0$ on a smooth horizontal surface strikes and compresses a spring of stiffness $k$ until the 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
According to the observer $A$:

  • A
    the kinetic energy of the block is converted into the potential energy of the spring
  • B
    the mechanical energy of the spring-mass system is conserved
  • C
    the block loses its kinetic energy because of the negative work done by the conservative force of spring
  • D
    all the above

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