ARISTOTLE’S FALLACY

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.