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2014-11-18T23:45:22+05:30

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There is only one force acting on the block.

The work done on the block of mass 2 kg by the friction force F in stopping it from a velocity of 2 m/s through a displacement S = 20 cm is:  W
  
         v² - u² = 2 a S,      v = 0     =>   - u² = 2 a S
    => -1/2  m u² = m a S
    =>  -1/2 m u² = F S = W
 
           W = - kinetic energy at  t = 0
               = - 1/2 *2  * 2² = - 4 Joules.
        Work done is negative because the block moved against the force. The direction of force and displacement of the block are opposite.


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