Question

Describe the steps in solving the problem below.
A red cart of mass 4 kg slides to the right on a frictionless track with a velocity of +4 m/s. It collides with the stationary blue cart of mass 1 kg. After the collision, the blue cart has a velocity of +8 m/s. Calculate the velocity of the red cart after the collision. ;)

Answer 1

The velocity of the red cart after the collision is 2 m/s

From the law of conservation of momentum, initial momentum of system = final momentum of system.

m₁v₁ + m₂v₂ = m₁v₃ + m₂v₄ where m₁ = mass of red cart = 4 kg, v₁ = velocity of red cart before collision = + 4 m/s, v₃ = velocity of red cart after collision, m₂ = mass of blue cart = 1 kg, v₂ = velocity of blue cart before collision = 0 m/s (since it is initially at rest) and v₄ = velocity of blue cart after collision = + 8 m/s.

Substituting the values of the variables into the equation, we have,

m₁v₁ + m₂v₂ = m₁v₃ + m₂v₄

4 kg × 4 m/s + 1 kg × 0 m/s = 4v₃ + 1 kg × 8 m/s

16 kgm/s + 0 kgm/s =  4v₃ + 8 kgm/s

16 kgm/s =  4v₃ + 8 kgm/s

16 kgm/s - 8 kgm/s =  (4 kg)v₃

(4 kg)v₃ = 8 kgm/s

Divide both sides by 4 kg, we have

v₃ = 8 kgm/s ÷ 4 kg

v₃ = 2 m/s

The velocity of the red cart after the collision is 2 m/s.

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