Hi, I am an experienced programmer and both OpenGL and C++ belong to my skills. Also, I already have an idea for that scene. My suggestion would be showing a sphere attatched to a pendulum or a block attatched to a spring. This way you would show the simple, yet very interesting, example of harmonic motion (a classical behavior of physics). Besides, you would have something less trivial than spheres hitting each other, but if you still want gravity and collisions I can add to the program any way. By the way I have something like a spring already implemented in OpenGL.
Moreover, the extension of the spring could be controlled interactively. And just to add more realism, the oscillation could be damped (the oscillation has its amplitude progressively reduced until it ceases completely). If you're interested PM me, because I know exactly how to do all this. And of course, if you want proof of my skills with openGL, I can send you one or more programs (demo version) I've developed by using OpenGL.
In my propose, if you award me your project, I can send you, within three days, the implementation of the pendulum and the block attached to a spring. If you also want me to implement collisions and gravity I can send them to you within three days later than the first part of the project. If you have a tight deadline, just PM me so we can discuss a better price and deadline for you.
To sum up: The first part of the project would cost $50, while the other part would cost more $50.