Question

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.<br /><br />
NCERT Class IX<br />
Sciences - Main Course Book<br /><br />
Chapter 9. Force and Laws of Motion

Answer 1

   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