Principal of Relativity
Imagine you are sitting on a park bench watching a bird gently fly overhead where the bird is traveling in a linear uniform motion relative to your “stationary” motion. Now imagine that same bird but as you watch it from a car that is also traveling in a uniform linear motion. In both cases the bird will have been traveling in a straight linear velocity because the observer was either “at rest”, like sitting on a park bench, or also moving in a linear velocity, like in the car.
In both cases the laws of physics as determined by men like Newton will work perfectly well. This is because there is no absolutely position as to observe nature, everything is relative. If this simple principal of relativity was no true then the math in physics would change depending on the time of year because the speed of Earth’s rotation around the sun will slightly vary, this is obviously not the case.