12V, 100W DC Motor speed is 2500rpm, gear ratio 50:1, hence at output shaft speed is 50 rpm.

Wheel diameter is 10cm.

I know that, Torque = Force x length of lever arm.

But, when wheel is directly connected to motor shaft, what to put in 'length of lever arm' ?

Is it the wheel radius?

How to calculate motor torque required for this requirement?