Answer
The tenth car pulls the eleventh car with a force of $100,000~N$
Work Step by Step
We can find the acceleration of the train:
$v_f=v_0+at$
$a = \frac{v_f-v_0}{t}$
$a = \frac{4.0~m/s-0}{20.0~s}$
$a = 0.20~m/s^2$
We can consider the system of ten cars from car 11 to car 20. The force that car 10 exerts on car 11 accelerates this system. We can find the force $F$ that car 10 exerts on car 11:
$F = ma$
$F = (10\times 5.0\times 10^4~kg)(0.20~m/s^2)$
$F = 100,000~N$
The tenth car pulls the eleventh car with a force of $100,000~N$
