Chapter 4 / 4.2 Aristotle’s Fallacy

Aristotle believed an external force is needed to keep a body in motion. For example, he thought air pushes an arrow forward after it’s shot.
This idea seems intuitive because everyday objects (like toy cars) stop moving without external force. However, Aristotle overlooked friction as the opposing force.
In reality:
- Objects slow down due to forces like friction or air resistance.
- If friction is absent, no external force is needed to maintain motion. For example, a toy car on a frictionless floor would keep moving forever once set in motion.

Galileo challenged Aristotle by studying motion on inclined planes:
- Objects accelerate downward on a slope and decelerate upward.
- On a horizontal frictionless plane, objects move at constant speed: \( v = \text{constant} \).
He used a double inclined plane experiment:
- A ball rolling down one side climbs up the other, nearly reaching its original height.
- If the second slope is horizontal, the ball would roll forever. This idealized case led to the concept of inertia.
Key conclusion: A body in motion stays in motion with constant speed unless acted upon by a net external force.

Newton’s First Law (Law of Inertia): Objects maintain their state of motion unless a net force acts. Formula: \( \sum F = 0 \implies v = \text{constant} \).
Role of Friction: Explains why Aristotle’s idea fails. Friction opposes motion, requiring external forces to maintain speed.
Galileo’s Experiments: Inclined planes and idealized frictionless motion led to the law of inertia.
Aristotle’s Fallacy: Misinterpreting everyday friction as a need for constant force.