# Forces in objects

1. Jun 6, 2014

### vaart

I had a question that was on my mind for a while. The general question is about structures and a force on that object in a certain direction and how the entire structure is accelerated by this force.

An example: If I act a force on two trains with equal mass that are attached to each other, both trains acceleration will be the same. What forces act on the suspension point of the trains? I know the total force can't be zero, because the suspension point is also accelerating.
This is just a small example, but in high school I only got examples of how forces that acted on whole objects and not the effect on multiple objects.

Thanks,

vaart

BTW, this is my first post on PF, so please let me know if I didn't do something right. And offcourse: thanks for you help in advance!

2. Jun 6, 2014

### Philip Wood

Suppose the last carriage of a train is being pulled by its coupling to the rest of the train. As you say, all parts of the train, including the back carriage, will have the same acceleration, a.

Then if the mass of the back carriage is m, and if there were no resistive forces on the carriage, the pull exerted by the coupling on the back carriage would be ma. If there is a resistive force, R, acting in a backwards direction on the carriage, then the force exerted by the coupling on the back carriage will be ma + R. Using Newton's third law, the coupling (forgetting its own mass and acceleration) will exert a backward force of magnitude R +ma on the last-but-one carriage.

That's the sort of reasoning you can use to understand 'internal forces'.

3. Jun 6, 2014

### vaart

Thanks! I think I just saw it to complicated. If I understand you correctly, all the particles in the train has the same acceleration and if there is another force acting on the whole train, all the particle will be affected accordingly.
The reason I came up with this question is that when I thought that if a force acted on 1 train with one wagon, it would accelerate faster than the same force on a train with 2 wagons. I thought the accelaretion was lower because the second wagon was acting a force on the second, but now I realize that it's just that the larger mass has a larger resistance to move. I think I am on the right track now, thanks for that! and if not, please let me know :)

4. Jun 6, 2014

### Philip Wood

Yes, I think we agree.

A piece of advice... Objects exert forces on each other. If you are considering the forces on an object, always make sure you can identify the external object which is exerting each force. If you can't, you may not really have a force at all!

5. Jun 7, 2014

### vaart

Thanks for the advice! And thanks in overall, my first FP experience has been a pleasent one, I will be surely posting more question that are troubling me in the future :D