Originally Posted by
JC
There is something about your initial descriptions that bother me, and i think it affects your analysis, but i'm not sure if i'm "getting" it so i'll try and refute your explanation and see if it sticks i guess!
In your first part, you describe the system as under no gravity and as internal-only forces. This is fine, and describes the movement in free floating space (for simplicity i imagine).
You then go on to say that because of this, there can be no torque at A because G is also free floating and operating as an internal force, which is still fine, you've added a torso to your space arm model.
Where i think my problem lies is here; there is an additional component missing underneath G, the bench itself. And without the bench, the torso rotates about point A (free floaing in space and all), which is why it has no external forces or torques acting there. But by adding a bench underneath G, it can no longer rotate at A and this creates the external force required for the "floating space arm" to push up at what becomes point C.
Also, it means that in your initial model, either you need to add in the bench and it's external force at A, or else in diagram "time 2" you need to alter G to show it sloping down from left to right as it is operating under internal forces only with no torque at A.