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Figure 1.2 motion of an object along a curve. Credit: “Physics for scientists and engineers strategic approach” by
Randall D. Knight, Pearson-Addison Wesley 2004
1.2 Aristotle and Galileo Views of Motion
The Greek philosopher Aristotle (4th Century BC) studied motion and divided it into to two kinds
1. Natural motion
Motion that is caused by no external forces. For example, the downward motion of a falling stone
and the rise of smoke
Heavy objects fall faster than light objects
The state of rest is the only equilibrium state. All moving objects will end up at rest
2. Violent motion
Motion that is caused by an external force. For Aristotle, the force is any push or pull on an
object. He thought, objects do not move unless a force act on them. For example, the motion of a
cart pulled by a horse and the motion of an arrow by a stretched bow
Aristotle used rational thinking and not experimentation as a key to describe and understand motion.
His view captured people attention for over 2000 years (until 16th century).
For Galileo (16th Century), on the other hand, both rational thinking and experimentation were the
keys to describe and understand motion. He developed his own views of motion, which can be
outlined by the following
Forces act through a physical contact (push or pull) or non-contact (at a distance, gravitational
force for example) to move objects. Notice that a non contact force is Galileo‟s term
Forces change object‟s velocity or direction over time. A force cause an object initial at rest move
(throwing a ball, for example) or to stop it if it was moving (catching a ball, for example). Forces
can also cause a moving object to change its direction (hitting a moving ball with a bat, for
example)