> Why put the flywheel close to the head?

Just thought it was good imagery. It didn't really matter if it's at the hips or head for that model.

> 4kg flywheel spinning at, usually, 2000 to 6000 RPM

Assuming a 12 inch flywheel and 6000 RPM, that's roughly equivalent to 4kg moving at 200MPH. Roughly 2% of the kinetic energy of a subcompact moving at 25mph. So I mean, it's not incredibly significant.

Anyways, we all took the same physics classes as you. Nobody is arguing with you that rotational energy doesn't effect linear motion in an inertial reference frame in a frictionless vacuum.

> We don't lean forwards just to walk

We definitely shift our center of mass in the direction of acceleration to accelerate. You don't have to call it leaning, and you could implement a robot whose torso remains upright while shifting its center of mass forwards, but you have to agree with me on this one.

I feel like we're having different arguments here. I'm trying to come up with some plausible model of what klienbl00 could have meant when he said a flywheel could resist linear motion. Obviously our high school physics knowledge makes that sound implausible, but I was having fun trying to come up with an explanation other than "no you're wrong".

I was frustrated by statements like "the body would have no trouble applying the force necessary" and "doesn't create as much torque as you believe", because I was shooting for an explanation of any possible resistance, not just sufficient resistance to make a flywheel-balanced bipedal robot infeasible.