Mistake 1- Static friction:
A reader pointed out that static friction is the key to scooting. I do not think this is correct (its not the key). However, I did realize that I should have included it in my calculations. So, I re-wrote the program to include that. Here is the new data I get:

But, in creating this graph I realized my second mistake.

Mistake-2 Force on the top mass:
Here is the basic idea behind my program:

• Create a "driving force" on the top mass of the form F=A cos(ω t)
• Treat this "driving force" as an internal force between the top and bottom mass. This allows me to calculate the resulting force on the bottom mass
• Now use my normal numerical methods for both the top and bottom mass (with static AND kinetic friction)

So, what is the problem? (that is exactly what I originally thought). The problem is that the force on the top mass is only time dependent. This causes the top mass to oscillation with respect to a fixed position and not with respect to the bottom mass. The physics is still valid, just not as realistic. If you look at the above graph, you will see that the top mass keeps returning to the origin while the bottom mass moves forward. I will try to fix this problem.

Tags: forces, kinematics, modeling, momentum