A girl of mass 40 kg jumps with a horizontal velocity of 5 m s-1 onto a stationary cart with frictionless wheels. The mass of the cart is 3 kg. What is her velocity as the cart starts moving? Assume that there is no external unbalanced force working in the horizontal direction.

NCERT Class IX
Sciences - Main Course Book

Chapter 9. Force and Laws of Motion

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2015-05-21T15:11:15+05:30

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   no external force  =>  momentum is constant.

   momentum of girl = 40 * 5 = 200 kg-m/sec
  momentum of girl and cart = total momentum of the girl + momentum of cart
         = 200 + 0 = 200 kg m/sec
     speed     = 200 / (40 + 3) 
             = 4.65 m/sec
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