Hi all. First post so please forgive the noobness. I have been listing to an audio book of "A briefer history of time" by that well known genital man and I have hit upon something I don't understand. My understanding of the Principle of Equivalence is that you will not be able to tell if you are in a rocket ship accelerating in space or a room that is at rest in a gravitational field on Earth. From this we are supposed to deduce there is no observable distinction between inertial motion and motion under the influence of the gravitational force. The thing I don't understand is; I thought that inertial motion and accelerating motion are different. If the rocket ship is accelerating and the resulting effect is a gradual strengthening of force upon the spaceman, it would continue to grow stronger until the force overwhelmed the poor fellow and we would run into the "mass can't reach the speed of light" issue. On the other hand my understanding of inertia is that it will remain constant until a force acts upon it. How can the illustration of a force which require constant acceleration show how there could be no observable difference between an inertial motion and a motion under the influence of the gravitational force? I'm sure I have missed something obvious. Thanks in advance.