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# Mechanics- Connected Particles- When To Seperate? watch

1. Hi, is there a clear cut rule for when to seperate particles and resolve them, and when to treat them together (like the typical car towing a caravan type questions). I just had a question and part c involved an incline and someone told me you treat them as one particle, but when no incline you treat them seperately??????????
2. Give an example.

Newton.
3. question 6 in attachment
Attached Files
4. question 6.doc (64.0 KB, 95 views)
5. i sucessfully did a and b by seperating the particles and resolving seperately, but i did c got it wrong and the guy from ajamal.com has worked it out as one particle..........
6. anyone?
7. What answers did you get?
8. (Original post by Nima)
Get the paper from here:

http://www.mathsexams.ukteachers.com...1_2001_Jun.pdf

Question 6

i worked out a and b by seperating the objects, but for c the guy from ajamal.com combined them........????
9. it depends on what you are trying to find. I f you solve as a whole sometimes opposite froces cancel each other out which makes it easier to solve questions. It is best to solve seperately the one which has the least forces on so you do less work and it is easier to reach a solution
10. You treat them as one in this case because it says AND. Plus you know that the towbar is inextensible so that means that the tension throughought it shall be equal and so shall the acceleration. So you can treat them as one.

Newton.
11. (Original post by faa)
Hi, is there a clear cut rule for when to seperate particles and resolve them, and when to treat them together (like the typical car towing a caravan type questions)....
It depends on what you're trying to work out.
Sometimes it's easier to treat it as a system problem, sometimes easier to treat it as applying to only one part of the system

In part (a), you should combine the particles into a single system and solve.
The fact that the question asks you for the acceleration of the system (car and van) is a good indicator of this.
The mass of the system is the sum of the two masses.
The resistance to motion of the system is the sum of the resistances to motion of the car and the van.
The force being applied to the system is the driving force from the van since the van and car are attached by an inextensible rod.

In part (b), you are asked about the tension in the rod. The rod is an internal part of the system, so you only consider one of the particles (the car).
The mass of the car is given.
The resistance to motion is now only the restance to motion of the car.
The driving force on the car is still what it was before, but this time you subtract the tension in the rod, since that is acting against the motion of the car.

In part (c), it is treated as a system again regardless of whether it is on an incline or not. You are asked to find the accln of the system (car + van) and all other forces and weights can be made applicable to the system as a whole.

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Updated: December 28, 2004
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