hi ,

Please explain how much torque needed for a DC motor to move/rotate at constantly slow pace( a = 0 m/s^2) of a platform with radius 15cm and load 3kg( on top of the platform, weight distributed evenly). The DC motor is located below this platform, and geared in its middle to spin it.

Thanks

Depending on how well the bearings are designed, the answer is not much torque is needed at all. Any reasonable battery-powered electric motor could be geared to make this work. Is there a specific motor you have in mind?