Sizing a motor for a conveyor
The torque doesn't have anything to do with accelerating the motor, it's much more about accelerating the load. How fast is your belt moving?

You say you have 6000 lb &mdash; presumably that's the total load on the 20 feet of belt at any given time. Suppose your belt is moving at 2 feet/second &mdash; that means that any given bit of load spends 10 seconds on the belt, and that you're adding 600 lb/second to what the belt is carrying. THAT is the mass that needs to be accelerated from 0 to 2 fps, requiring torque from the motor to do so.

Multiply those two numbers together, and you get 1200 ft-lb/sec2, which is a direct measurement of the force on the belt. Multiply by the radius of the drive roller in order to get the torque, and from there you can figure out the gearing to match it to motor torque and speed (and therefore power).

There are other power losses associated with moving the belt, so incorporate a design margin into your final selection. If your conveyor is not level, then you also need to add the force required to lift the load against gravity to your calculation.