Momentum is the thing. In Physics, a body that is in motion has momentum proportional to its mass and velocity. Motion is vital – if the velocity is zero, there is no momentum.
Newton’s First Law of Motion states that ‘a body at rest will remain at rest, and a body in motion will remain in motion, unless it is acted upon by an external force.’ Basically, the second part of this Law means that, in a vacuum and frictionless setting, once something is on the move it will stay that way forever, unless one actively inputs energy to stop it.
Clearly, far from vacuums and ideal theoretical states, we don’t live in a frictionless world. There are stoppers all around us. Thus, any momentum we achieve will wane unless we deliberately inject further (and continuing) energy into the system. To keep up the forward motion we have to keep working.
That may sound exhausting. I don’t mean it to. It’s just a reminder for me today that coasting for longer than the indeterminate ‘little while’ isn’t possible. One needs to pedal that bicycle at least occasionally. That steam train needs its fire.