if u hold a shopping on a cart on a treadmill, the wheels of the cart will be going say 10mph while the treadmill goes -10mph. the cart however, does not move. if u accelerate the treadmill to -20mph, the wheels of the cart will spin at 20mph but the cart will still not move. therefore it doesnt matter what speed the treadmill goes because the wheels are free spinning. so now that u have a cart not moving, but rolling along with the treadmill, put jet engines on it. will the cart move forward? get it?