>>11462345To put it simply as possible:
Imagine a vacuum in a gravitation-less environment.
Place a body of some mass in that space. In order to move that body you have to exert some force on it.
Lets say the mass is 1kg. And you "push" the body with 1N of force for a total of 1 second.
That mass would gain a acceleration of 1m/s^2. Since the derivative of acceleration in time is speed, in a gravitational-less vacuum the body would continue to move at the speed of 1m/s forever.
Standing still or moving at a constant speed is indistinguishable for the body of a certain mass. Body can only "detect" change of speed, which is acceleration by definition.
When the body is moving it has a certain kinetic energy that is proportional to mass and velocity, but that is another subject.
