calculating torque in robot arm

[Mahmoud]

please every one  
can any one help me
in robot arm tutorial calculating torque it says that (if J0 was additionally affected by gravity, add the torque required to lift the arm to the torque required to reach the velocity you need) i do not understand why and how. the equation torque = angular acceleration*moment of inertia i think it is for all to  keep the arm stationary and to accelerate the weight being support by an actuator from a static position
 

[Admin]

Torque is required to rotate a mass, simply because of inertia. (for a non-stationary arm only)

When gravity pulls on the mass, torque is also needed to counter that acceleration.

If your robot arm is rotating on Earth, it'll need to counter both inertia and gravity. So you calculate each individually, then add them up.