This might be the dumbest question of the year, but, can I run a 6V motor (https://www.pololu.com/product/3451) with a 5V battery? Would I need to use an op amp to increase the voltage from the battery to 6v?
Also, can this motor be run on 1.2A continuous? I saw that stall was 2.9A and it recommended .25% of stall, so that’d be around .75V.
5V is an unusual nominal battery voltage. What particular battery do you have?
You can generally power brushed DC motors (like the one you linked to) with voltages lower than their nominal ratings as long, they will just output proportionally less speed and torque than listed in the motor specifications. At some point the supply voltage could be too low for the motor to start up, but powering a 6V motor with 5V should not be a problem.
If you want to increase the speed and torque of your motor then you could do that using an appropriate step-up voltage regulator.
I am not sure I understand your second post. Where are the 0.75V and 1.2A continuous current values you mentioned coming from? How much current a brushed DC motor draws (or at least tries to draw) is a function of the applied voltage and the load on the motor. That relationship is shown by the performance curves in our Datasheet for Pololu 20D Metal Gearmotors (which is available one each 20D gearmotor’s product page under the “Resources” tab).