Question

A newspaper boy is coasting forward at 3 m/s on his skateboard. Boy, newspapers and skateboard have a mass of 35 kg. He tosses a 1 kg newspaper directly back over his shoulder with a horizontal velocity of 9 m/s (relative to the ground). How fast is he going then?

Answer 1

Mass of the boy, newspapers and skateboard = m₁ = 35 kg

Mass of the newspaper = m₂ = 1 kg

Initial velocity of the newspaper boy = V₁ = 3 m/s

Velocity of the newspaper after the boy throws it = V₂ = -9 m/s (Negative as it is thrown backwards)

Velocity of the boy and skateboard after he throws the newspaper = V₃

By conservation of linear momentum,

m₁V₁ = m₂V₂ + (m₁ - m₂)V₃

(35)(3) = (1)(-9) + (35 - 1)V₃

V₃ = 3.35 m/s

Speed of the boy after he throws the newspaper = 3.35 m/s