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.

NCERT Class IX
Sciences - Main Course Book

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

Answer 2

Suppose the velocity of the boy on the cart as the cart starts moving is V

Therefore the total momentum of the boy and the cart before the interaction

Equal 50 kg into 5 metre per second + 3 kg in 20 metre per second

Equal 200 kg metre per second

Also the total Momentum after the interaction is equal to 40 + 3 into b is equal to 43 kg metre per second

As per the law of conservation of momentum the total Momentum is conserved during the interaction in other words 43 V is equal to 200 V is equal to 243 that is equal to 4.65 M per second

Does the boy and the God would move with a velocity of 4.65 metre per second in the direction in which the boy jump down to the cart