I came across this question, " An 8000 kg engine pulls a train of 5 wagons, each of 2000 kg along a horizontal track. If the engine exerts a force of 40000 N and the track offers a frictions of 5000 N, then calculate the force exerted by wagon 1 on wagon 2." In this question,if we consider wagon 2 the system, the horizontal forces would be the tension T of the string by which it is attached to wagon 1 in the left direction, and tension T1 of the strig by which it is attached to the rest of the wagons in the right direction. So the force would be mass * acceleration which will be equal to T -T1. My question is that why do we need to consider the mass of the rest of the wagons 2,3,4 and 5 too? It is our choice, we can take any number of bodies or any body as a system;here I have taken wagon 2 as a system, so why do I need to consider the mass of the other wagons? We had this question in our school textbook and when our teacher solved it, she considered the mass of the wagon 2 + wagon 3+wagon 4+ wagon 5. She might have chosen wagon 2,3,4,5 to be a system, but I have chosen wagon 2 as the system. Please help!