In Newton's dreams...

Graphic - newtonsdreams

Like all laws of Physics Newton’s 1st Law describes an idealised world to which the real world does, at best, approximate.

The conditions prescribed in the 1st law would be realised if:

  1. the reference frame was a remote star, free from the influence of anything else in the universe;

    and

  2. the body was another ‘remote star’, again free from the influence of anything else …including the star defining the reference frame.

In practice the earth (and thus a bench fixed in the laboratory) provides a good approximation to an inertial reference frame; and a puck on an air-table on the bench provides a good approximation to a body free from (net) external forces.