# My confusion regarding Force

1. Sep 3, 2011

### AakashPandita

There is something that I not able to understand about force. I would like to tell it to you with an example.
There is a train consisting of 1 engine of 8000kg and 5 wagons of 2000 kg each
The acceleration of the train is 2 m/s2

How would you tell what force is exerted by wagon 1 on wagon 2 and why?

2. Sep 3, 2011

### stallionx

1<->2<->3<->4<->5->

F exerted to the ultimate wagon on the line

look at it as a whole

Find acceleration of the train and then look at 1<->2 ( Forces act equal but in different directions in each case )

What do you guess the acceleration of the first wagon be related to overall acceleration.

Good Luck.

3. Sep 3, 2011

### AakashPandita

Actually what i did not understand was....
we need to find force by 1 on 2........
but 1 is connected to engine and 2 is connected to wagon 3,4, and 5.
And according to newton's 3rd law,
the force exerted by engine and wagon1 should be equal to the force exerted by wagons 2,3,4,5 in opp. direction
i.e,
ma=-ma
(8000+2000) x 2 should be equal to -(2000 x 4) x 2
but it is not so...

it is difficult to understand my question but if you understand...please tell me the reason....where am i getting wrong?

4. Sep 3, 2011

### stallionx

Mtotal * a = what is it?

5. Sep 3, 2011

### tiny-tim

Hi Aakash!
That's right!

You have correctly identified the two bodies whose equations of motion you need to consider.
No, you need two equations, one for each body.

For the rear body, yes, it's F = mreara.

But for the front body, it's force-from-the-engine - F = mfronta.

So how do you find force-from-the-engine ?

6. Sep 3, 2011

### AakashPandita

First of all, For the front body what is "F" that you mentioned?
And...If the equation of the rear was right then shouldn't it be equal to (m front a)?
And why can' t the equation for the front be written as the product of its mass and a?
Why is it wrong?

7. Sep 3, 2011

### stallionx

Look at the interaction between 4 and 5

1,2,3,4(4 masses ) -> some force

that some force <-5-> Force total

Drawing FBD's what do you arrive at ?

8. Sep 3, 2011

### AakashPandita

i understand your point.
but i don't find the answer to my question which is in the 3 rd post of this forum.

9. Sep 3, 2011

### tiny-tim

Hi Aakash!
For both bodies, F is the equal and opposite forces between the front and back parts.

Now take a closer look at the question …
i] what is meant by the word "engine"?

ii] why is the whole train accelerating??

10. Sep 3, 2011

### AakashPandita

engine is pulling all the wagons, i.e, applying a force on them and so it is accelerating.

11. Sep 3, 2011

### AakashPandita

please tell me what mass should we multiply with acceleration.......mass of wagon1+ that of engine...................or mass of wagon2+3+4+5.

12. Sep 4, 2011

### tiny-tim

Hi Aakash!

(just got up :zzz: …)
Exactly

the question is telling you (without actually saying so) that there is an extra force (call it E).​

So how are you going to include E in your equations?
You always use the mass of whatever "body" your equation is about.

And you need a separate equation for each body.

13. Sep 4, 2011

### AakashPandita

i dont understand what you are trying to say.

14. Sep 4, 2011

### tiny-tim

which part?

15. Sep 4, 2011

### AakashPandita

the answer for my question( which you have quoted)

16. Sep 4, 2011

### Staff: Mentor

To apply Newton's 2nd law:
Pick the body (or group of bodies) you want to consider as your system.
Identify all the forces acting on that body.
Find the net force on the body.
Then apply ΣF = ma, where m is the mass of whatever 'body' you are analyzing.

17. Sep 4, 2011

### AakashPandita

what i do not understand is if wagon 1 is exerting force on the 2nd what mass should be multiplied with acceleration...........
engine+wagon1.......or mass of wagon2+3+4+5

18. Sep 4, 2011

### Staff: Mentor

First tell me:
What forces act on 'engine+wagon1'?
What forces act on 'wagon2+3+4+5'?

19. Sep 4, 2011

### Staff: Mentor

Another hint if you need it: What's producing the acceleration of the entire train (including all the cars)?

20. Sep 5, 2011

### AakashPandita

the force acting in both the cases should have the same magnitude...... shouldn't it?

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