4Force and Motion
If I have seen farther than others, it is because I have stood on the shoul-
ders of giants.
Newton, referring to Galileo
We need only explain changes in motion, not motion itself
So far you’ve studied the measurement of motion in some detail, but
not the reasons why a certain object would move in a certain way. This
chapter deals with the “why” questions. Aristotle’s ideas about the causes of
motion were completely wrong, just like all his other ideas about physical
science, but it will be instructive to start with them, because they amount to
a road map of modern students’ incorrect preconceptions.
Aristotle thought he needed to explain both why motion occurs and
why motion might change. Newton inherited from Galileo the important
counter-Aristotelian idea that motion needs no explanation, that it is only
changes in motion that require a physical cause.
Aristotle gave three reasons for motion:
• Natural motion, such as falling, came from the tendency of objects
to go to their “natural” place, on the ground, and come to rest.
• Voluntary motion was the type of motion exhibited by animals,
which moved because they chose to.
• Forced motion occurred when an object was acted on by some
other object that made it move.
Even as great and skeptical a genius as Galileo was unable to
make much progress on the causes of motion. It was not until a
generation later that Isaac Newton (1642-1727) was able to attack
the problem successfully. In many ways, Newton’s personality was
the opposite of Galileo’s. Where Galileo agressively publicized his
ideas, Newton had to be coaxed by his friends into publishing a book
on his physical discoveries. Where Galileo’s writing had been popular
and dramatic, Newton originated the stilted, impersonal style that most
people think is standard for scientific writing. (Scientific journals today
encourage a less ponderous style, and papers are often written in the
first person.) Galileo’s talent for arousing animosity among the rich
and powerful was matched by Newton’s skill at making himself a
popular visitor at court. Galileo narrowly escaped being burned at the
stake, while Newton had the good fortune of being on the winning
side of the revolution that replaced King James II with William and
Mary of Orange, leading to a lucrative post running the English royal
Newton discovered the relationship between force and motion,
and revolutionized our view of the universe by showing that the same
physical laws applied to all matter, whether living or nonliving, on or
off of our planet’s surface. His book on force and motion, the
Mathematical Principles of Natural Philosophy, was uncontradicted
by experiment for 200 years, but his other main work, Optics, was on
the wrong track due to his conviction that light was composed of
particles rather than waves. Newton was also an avid alchemist, an
embarrassing fact that modern scientists would like to forget.
Aristotle said motion had to be caused
by a force. To explain why an arrow
kept flying after the bowstring was no
longer pushing on it, he said the air
rushed around behind the arrow and
pushed it forward. We know this is
wrong, because an arrow shot in a
vacuum chamber does not instantly
drop to the floor as it leaves the bow.
Galileo and Newton realized that a
force would only be needed to change
the arrow’s motion, not to make its