Question

how can you verify newtons first law of motion

Answer 1

This is most easily proven by contradiction. For example, imagine we are in a vacuum. If Newton's 1st law were not true than if I were to push a ball forwards than in theory it could at any point stop, change direction, or any number of things. However, we know this is not the case. When I push a ball in a vacuum (let's just say "in space") we see that the ball continues along a single path. Until something else hits or acts to slow it down, the ball will continue on its path at a constant velocity or acceleration.

Answer 2

Not a constant acceleration, just a constant velocity. A constant acceleration would only occur if a constant force were applied.

You can test how accurate Newton's first law of motion is by entering into a freely falling frame on earth or by getting as far away from massive objects as possible and then giving a free mass an initial velocity. If its velocity does not vary to any measurable extent along its trajectory then Newton's first law can be deemed at least as accurate as implied by the measurement uncertainty of the experiment.

Answer 3

I meant to say of course that this projectile mass needs to be in a vacuum.