I have a slide which I pull along the airtrack with a string. While I do not
care about the absolute magnitude of the force I can reproduce
the magnitude by extending the spring by the same exact distance.
As we shall discuss later when extended, a spring produces a force
According to
Newton's second law if the force is constant I should get constant
acceleration
and that acceleration decreases inversely in proportion with the
mass m of the vehicle. I pull vehicles with different masses
along the air-track
with constant force (constant extension of the spring)
and you see that the
acceleration is greater for the light than for the
heavy vehicle. This is the basic qualitative content of Newtons second law
of motion.