Haven't read all the details yet, but from the photo your DC Motor don't have a Gear box fitted to them (and from the looks of the motors they don't have a reduction built in) so they will not have enough torque to drive a platform. Have a search for a geared motor (often they either come with wheel attachments or a coupler for the wheels if seperate). Ungeared they spin at a rate of knots but have no grunt to drive a wheel which will experience resistance...
Yeah, when they are powered up, it doesn't take much to stop them. The original project I bought them for didn't require much, though. If I ever decide to build the "car" project, I will definitely look into a more capable motor.
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