Five train cars and their final speed

[brunettegurl]

Homework Statement

Three identical train cars, coupled together, are rolling east at 1.13 m/s. A fourth car traveling east at 6.43 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. All 5 cars are identical. What is the speed of the five-car train?

Homework Equations

p_initial = p_final
m1v1+m2v2 = m_totalv_final

The Attempt at a Solution

so this is what i did

3mV+ mV= 4mV [the masses cancel]
(1.13)+(6.43)= V
7.56 m/s = v

4mV+ mv = 5mv
Vtotal+0 = vfinal
7.56 m/s = vfinal

can someone pls tell me what I'm doing wrong thanks

 

[nickjer]

Looks like this exact same problem was recently discussed here:

https://www.physicsforums.com/showthread.php?t=155891

I haven't read it, but it might help quite a bit. I also recommend you label your velocities like v1, v2, etc.

Edit: Seems you noticed it already, I see your post in there now.

 

[nickjer]

You do not add the velocities. You use the conservation of momentum equation. So for the first part you would have...

3m*(1.13 m/s) + m*(6.43 m/s) = 4m*v_final

Then you solve for v_final. You do the same for the last part of the problem.

 

[brunettegurl]

thank u sooo much i feel so foolish that was really straighforward..thanx again