For a single rigid link of mass m, length L, rotating about a fixed axis, the moment of inertia is I = (1/3)·m·L². What is the torque required to produce an angular acceleration α?
Aτ = m·L·α
Bτ = I·α = (1/3)·m·L²·α
Cτ = m·g·L (gravity only)
DInsufficient information; the moment of inertia depends on the shape, not just mass and length
Gravity compensation is configuration-dependent. Sophisticated robot controllers measure the gravity torque at each configuration and actively compensate for it. Without compensation, the robot would sag under gravity when held stationary. With compensation, the robot feels 'weightless' to the operator during manual manipulation (teach pendant programming).