Okay if I was in an inertial frame of reference where there are no forces involved. If I were throw a pen by exerting a force on it, I would give it an acceleration of lets say 1 m/s^2 F = MA A = F / M So say that pen drifted of into space forever, wouldn't it eventually accelerate to 99% the speed of light since it has an infinite amount of time to accelerate? If there are no other forces on the pen. Wouldn't the acceleration eventually stop, even if there are no other forces? and what determines when it will stop accelerating?