John Dreher wrote:However I fear that the mechanics of the collision between two rolling spheres, with allowance for the assumed interaction at the contact point of the impact with regards to friction, puts the theoretical basis well beyond grade 6-8 level
With an appropriate choice of hypothesis and initial test conditions, one can usually ignore the small amount of friction between a sphere and a flat level surface and the small amount of elastic behavior at contact and air fluid flow and all the other complicating factors. Engineers make these simplifying assumptions all the time, they just have to go to the trouble of calculating the maximum error that could be caused by the simplification if they don't want to end up with a nasty surprise of being VERY WRONG in their predictions. For a science project, your hypothesis doesn't have to be correct! It has to be testable. A lot of great scientific discoveries have occured when somebody's hypothesis was WRONG! Many times actually proving that you have actually proved a hypotheisis is correct is EXTREMELY DIFFICULT. A good example of this is amount of effort that Michelson-Morley in their famous experiment.
If you start out with one stationary and one is moving object, that is a lot simpler than if both are moving. If you choose spheres as your objects, you eliminate a lot of geometry and effects of geometry. If you restrict your observations to the first few inches of travel before and after a collision, the effects of friction are less likely to accumulate and become significant. If you ignore the behavior 1 usec before and 1 usec after a collision, you won't have to try and deal with the precise contact behaviors and will only be left with the outcome of the collision.
The analysis of "dry-ice-sliders" can also get very complicated. Sublimation is occurring which is changing the mass so it is extremely difficult to know what the mass the slider is at all points in time. Yes there are again a set of simplifying assumptions that can be made, but again if you try and build a set of equations that fully includes everything, it can get far more complicated than interactions that don't involve state changes in matter. BEWARE: There is also a frost-bite safety hazard handling dry ice. Collisions of dry ice can send fragments in unexpected directions which means everone has to be fully protected head to toe, face shields, gloves, etc. so that dry ice fragments don't come in contact with any skin. Protective clothing needs to be simliar to the choice welder's make so that there aren't any pockets, cuffs, exposed sock/shoe openings where very hot (or in this case very cold) things can find their way into. Ava and Mom,
don't get discouraged by a couple of engineers/scientists discussing the complicating factors.
The fundamental baisis for Physics is simply coming up with an explaination for things that are observed.
If you get out your magnifying glass and look at every minute and complicating detail, you can make Physics really really hard.
If you stand back and look at things in less detail, it is far easier to come up with a testable hypothesis and experiment to test it.
At the 6-8 grade level, the first order effects are the important ones to be concerned with and not all of the minutia.