Question

A 30,000 kg railroad car initially traveling to the right at 15 m/s collides inelastically with a 20,000 kg railroad car initially traveling to the left at 5 m/s. The cars stick together. What is their final speed?

Answer 1

Question 18977

Lets use the law of conservation of linear momentum. Let the ox-axis go to the right. So, the projection of vector form $\vec{p}_1 + \vec{p}_2 = \vec{p}$ (where $\vec{p}_1$ is the linear momentum of the car with initial velocity $15\mathrm{m / s}$ , and $\vec{p}_2$ is the linear momentum of the car with initial velocity $5\mathrm{m / s}$ ) onto ox-axis is: $m_1v_0^1 - m_2v_0^2 = (m_1 + m_2)v$ , which gives

$$v = \frac {m _ {1} v _ {0} ^ {1} - m _ {2} v _ {0} ^ {2}}{m _ {1} + m _ {2}} = 7 m / s.$$