Consider the following situation. Two particles are initially separated by some distance and are then pulled further apart, thus increasing the potential energy of the system (due to the gravitational force of attraction between them). If I am correct in understanding special relativity, then the system's rest mass was initially less than that of the final system. So this should mean that it would be harder to accelerate the system of particles after they have been pulled farther apart than it was to accelerate them initially. In essence, you have to push on the particles harder to get them up to some given speed. In what ways has this been experimentally tested? Can anyone provide references to particular experiments or papers in which the experiments were described? I am quite intrigued by the fact that separating particles would somehow make them more resistant to changes in motion. Thanks for your information.