## Conceptual Physics (12th Edition)

An almost identical problem is done on page 101. Let m represent the mass of the freight car, 4m the mass of the diesel engine, and v the velocity after both are coupled together. Calculate the total momentum before the collision. $$(4m)(5 km/h) + 0$$ Now calculate the momentum after the collision, when the combined masses move together at velocity v. $$(4 m + m)v$$ By the conservation of momentum principle, the two expressions are equal. The mass m cancels out, and we solve for $$v = 4 km/h$$