Maximum tension between train cars, F=ma

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Homework Statement

A locomotive (6.4 × 10^5 kg) is used to pull two railway cars (Figure 11). Railway car 1 (5.0 × 10^5 kg) is attached to railway car 2 (3.6 × 10^ kg) by a locking mechanism.
A railway engineer tests the mechanism and estimates that it can only withstand 2.0 × 10^5 N of force. Determine the maximum acceleration of the train that does not break the locking mechanism. Explain your reasoning. Assume that friction is negligible.

open the link and scroll to question 7 to see the diagram.

http://www.lakeheadschools.ca/scvi_staff/childs/Gr11_physics_web/downloadable_content/unit3/textpdf3/phys11_3_5.pdf

Homework Equations

f = ma

The Attempt at a Solution

the only force on car 2 is the tension of the locking mechanism,
so Net force = 200kN

200kN / mass of the cart (360 000kg) = 0.55 m/s/s

However the answer key says that the answer is 0.23 m/s/s which is the answer if we considered the divided 200kN by 860 000 kg. I believe that that would be the answer if calculating the maximum acceleration given the maximum tension between the locomotive and car 1 instead of car 1 and car 2. Or have I mistaken?

 

[Doc Al]

However the answer key says that the answer is 0.23 m/s/s which is the answer if we considered the divided 200kN by 860 000 kg. I believe that that would be the answer if calculating the maximum acceleration given the maximum tension between the locomotive and car 1 instead of car 1 and car 2. Or have I mistaken?

You are correct. (There's no mention of any locking system between the locomotive and car 1.)