Three identical train cars, coupled together, are rolling east at 1.8 m/s . a fourth car traveling east at 4.5 m/s catches up with the three and couples to make a four-car train. a moment later, the train cars hit a fifth car that was at rest on the tracks, and it couples to make a five-car train. what is the speed of the five car train? answer in m/s.

Explanation:

consider the mass of each train car be m

m₁ = m₂ = m₃ = m

speed of the three identical train

u₁ = u₂ = u₃ = 1.8 m/s

m₄ = m             u₄ = 4.5 m/s

m₅ = m              u₅ = 0 (initial velocity )

final velocity

v₁ = v₂ = v₃ = v₄ = v₅ = v

using conservation of momentum

m₁u₁ + m₂u₂ + m₃u₃ + m₄u₄ + m₅u₅ = m₁v₁ + m₂v₂ + m₃v₃ + m₄v₄ + m₅v₅

m (1.8 + 1.8 + 1.8 +4.5) = 5 m v