# Homework Help: Objects connected, what is the force of one exerted on another

1. Sep 18, 2009

### katamoria

1. The problem statement, all variables and given/known data

A short train (engine + 4 cars) is accelerating at 1.1 m/s^2. if the mass of each car is 38000Kg, and if each car has negligible frictional forces acting on it, what are a) the force of the engine on the first car, b) force of the first car on the second car. and so on.
Note the importance of selecting the correct set of cars to isolate as your object.

2. Relevant equations

F=ma

3. The attempt at a solution

I've drawn a picture of the whole train travelling in the +x direction, and I've drawn the free body diagrams of each individual car with standard coordinate system, Normal and weight forces acting in the y directions, and the acceleration acting in the +x direction.
I know that there is no net force in any y direction. The only net force is the acceleration in x direction.
I'm probably making it too hard, but is it just plug in the mass of the car and the acceleration? does it make no difference where in the line up the car is, the force acted on each car by the car in front of it is the same for each one, right?

2. Sep 18, 2009

### physicsface

Hm... I'm not quite sure, but I'm pretty sure that you need to take into account the car in question as well as every car before it. Because if you're looking for, say, the force on car 3 exerted by car 2... Well, car 2 is being acted on by both the engine and car 1, so the force accumulates. I think you need to use the mass of the entire train from the engine back to the car in question.

3. Sep 18, 2009

okay. ^.^