[EricH]

Originally Posted by gblake

Erich - Don't forget to remind your audience that inertia exists, and that it makes a moving robot do things that a stationary robot would not (like tip over even if the CoG is within its wheelbase)...

Oh, Newton's first law?

That is true, especially if there's a sudden stop or change of angle involved. See, in physics (and in Statics), you can usually get a really, really good approximation of forces and how they'll react with each other by finding the CG and having all the forces act from that, unless you're dead sure that the force acts elsewhere (like friction, which gets as close as it can to the floor, or an impact from another robot, which should be within the bumper zone).

If you've got a robot traveling at constant speed, the only forces acting on it are weight, the normal force (cancels out the weight), friction (applied to the wheels) and whatever force you need to maintain equilibrium (applied to the wheels). Now, a sudden stop! The force needed to maintain equilibrium has been effectively distributed through the robot, and there is a sudden forward force at the CG (for simplicity--there's a rule saying I can move it there, I just forget which one) and a reverse force on the lower section of the frame. This creates a moment, causing the robot to rotate forwards. If a corrective force is not applied, the robot continues to rotate and goes splat. This effect is not helped if the CG is high, as there is now a much bigger "lever" for the forward force to act on, and it takes much more force to stop due to the way moment is calculated.