python ball physics simulation

I have seen the great tutorial by Peter Colling Ridge on
http://www.petercollingridge.co.uk/pygame-physics-simulation/
and I am extending the PyParticles script
The code is available on the site(for free), I am using PyParticles4.py

Classes used in the tutorial

The Particle Class
Circular 2d objects with radius,mass,velocity,location
The Spring Class
A spring that binds 2 objects (Particles) and uses the Hooke's law (F = -kx) to determine the interaction between them
The Environment Class
The Environment where the Particles interact

I was wondering if I could to use 2 Particles and make a 'Rod' class (like the Spring class in the tutorial) that had a specific length and didn't allow the particles to come closer go further than that (specified) length.
Also,
Appling a force (when needed) to each Particle such that if one is pulled toward the left, so does the other, but Realistically..
Much like if a 2 different types of balls were joined(from the center) using a steel rod, but in 2-d..
And I don't want to use 3rd party modules

Thanks in advance..

EDIT/UPDATE:
Tried to apply constraint theorem (it failed)
Here's the code:

class Rod:
    def __init__(self, p1, p2, length=50):
        self.p1 = p1
        self.p2 = p2
        self.length = length

    def update(self):
        'Updates The Rod and Particles'
        # Temp store of co-ords of Particles involved
        x1 = self.p1.x
        x2 = self.p2.x
        ###### Same for Y #######
        y1 = self.p1.y
        y2 = self.p2.y

        # Calculation of d1,d2,d3 and final values (x2,y2) 
        # from curr