For those not familiar with what a typical common-shaft motor-generator set entails: http://www.electrical4u.com/motor-generator-set-m-g-set/ Suppose an initially fully charged battery supplies the motor, a similar but initially depleted battery being charged up by the generator. Each battery adjacent to the respective motor, generator. Energy E thus a mass equivalent E/c^2 ends up being transferred from one location to another. But assuming the entire system is isolated, conservation of momentum requires the centre of mass is not changed by the transfer of mass-energy from battery to battery. At some stage then, one expects an axial force exists along the common shaft (assume a continuous one-piece shaft). Providing just enough impulse to keep the overall centre of mass fixed. The problem is how, or even if, that is possible given power transmission is via pure torsion in a rotating drive shaft. In more familiar cases; e.g. tandem-shaft belt or chain or gear transmission, or electrical transfer via wires, compensating forces are (relatively) easily identified. Not here. Looked at the issue many years ago, and iirc it took a good hour or so to arrive at an answer I was reasonably satisfied with. Instead of providing my answer(s) here and now, leaving it as is for ~ 24 hrs. Maybe somewhat more. A teaser challenge. Let's see who here can offer an answer with clear, credible justification, within that rough time frame. Just knowing that F = dp/dt will or should suggest what specific physical condition in above setup needs focusing on. Actually, there are two convenient limiting cases worth analyzing. The only hint offered is there are just two relevant variables to work with. Angular velocity and torque. Hand-wavy suggestions won't cut it. An acceptable answer(s) should ideally be quantitative not just qualitative. If anyone manages to or thinks they have found an essentially direct answer from a textbook or online article etc., be honest enough to admit it and reference to such. Good luck - back later.