Is an external force required to keep a body in uniform motion?
The question posed above appears to be simple.
However, it took ages to answer it. Indeed, the
correct answer to this question given by Galileo
in the seventeenth century was the foundation
of Newtonian mechanics, which signaled the
birth of modern science.
The Greek thinker, Aristotle (384 B.C– 322
B.C.), held the view that if a body is moving,
something external is required to keep it moving.
According to this view, for example, an arrow
shot from a bow keeps flying since the air behind
the arrow keeps pushing it. The view was part of
an elaborate framework of ideas developed by
Aristotle on the motion of bodies in the universe.
Most of the Aristotelian ideas on motion are now
known to be wrong and need not concern us.
For our purpose here, the Aristotelian law of
motion may be phrased thus: An external force
is required to keep a body in motion.
Aristotelian law of motion is flawed, as we shall
see. However, it is a natural view that anyone
would hold from common experience. Even a
small child playing with a simple (non-electric)
toy-car on a floor knows intuitively that it needs
to constantly drag the string attached to the toy-car with some force to keep it going. If it releases
the string, it comes to rest.
This experience is
common to most terrestrial motion. External
forces seem to be needed to keep bodies in
motion. Left to themselves, all bodies eventually
come to rest.
What is the flaw in Aristotle’s argument? The
answer is: a moving toy car comes to rest because
the external force of friction on the car by the floor
opposes its motion. To counter this force, the child
has to apply an external force on the car in the
direction of motion. When the car is in uniform
motion, there is no net external force acting on it:
the force by the child cancels the force ( friction)
by the floor. The corollary is: if there were no friction,
the child would not be required to apply any force
to keep the toy car in uniform motion.
The opposing forces such as friction (solids)
and viscous forces (for fluids) are always present
in the natural world. This explains why forces
by external agencies are necessary to overcome
the frictional forces to keep bodies in uniform
motion. Now we understand where Aristotle
went wrong. He coded this practical experience
in the form of a basic argument. To get at the
true law of nature for forces and motion, one has
to imagine a world in which uniform motion is
possible with no frictional forces opposing. This
is what Galileo did.