How much force is needed to accelerate a 50-kg rider and her 250-kg motorcycle at 5 m/s2?
This is really a physics question. The equation that you use is Newton's second law,
Force = Mass x Acceleration.
In this case, the total mass being accelerated is 300 kg. Multiply that by 5 m/s^2 and you get the force in Newtons.
(1 Newton = 0.224 pound-force)
Yeah, drwls is correct. The formula is Force = mass x Acceleration. It's a basic physics formula.