Posted by StressedStudent on Saturday, September 22, 2012 at 8:08pm.
A train consists of 50 cars, each of which has a mass of 7.2 × 103 kg. The train has an acceleration of +7.2 × 102 m/s2. Ignore friction and determine the tension in the coupling (a) between the 30th and 31st cars and (b) between the 49th and 50th cars.

s  bobpursley, Saturday, September 22, 2012 at 8:18pm
F=ma
so the tension in a coupling depends on the mass behind the coupling
In the case of the just last car,
tension=masscar*a
now between the 30and 31 car, there are 20 cars behind
tension=20(masscar)a