No.
Think of like a fly in your car. It flys around like it normally would, not at 55mph if the car is going 55mph. It flys at a relative speed to the car.
The ball already moving at 55mph relative to the ground, but zero mph relative to the truck. So it would be going 55mph off the back of the truck, and at the moment of the trigger firing the ball, the ball would go 55mph off the back end of the truck.
Not that hard to figure out.
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2/20/2013: They Day the ****s Stopped
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