1. Newton’s First Law of Motion –The Law of Inertia
Chapter 2
Newton’s First Law of Motion –The Law of Inertia

2. Objectives:
We will be able to compare and contrast Aristotle’s and Galileo’s ideas of motion.
We will be able to define and describe the concepts of speed and velocity.

3. 2.1 – Aristotle’s Classification of Motion
Aristotle believed that there are two types of motion -
Natural Motion
Unnatural Motion

4. 2.1 – Aristotle’s Classification of Motion - Continued
Natural motion – directed either up or down
Rocks to fall to the ground or smoke to rise in air
He believed that objects naturally seek resting places
Also believed that heavier objects fall faster than lighter objects
Believed it occurred w/o force
Celestial objects movements didn’t require forces

5. 2.1 – Aristotle’s Classification of Motion – Continued
Unnatural motion – requires forces
Such as forces imposed by animals or people

6. 2.2 – Galileo’s Concept of Inertia
Galileo proved Aristotle’s ideas to be untrue
There was one major difference between A and G
A used logic, or reasoned, thinking to establish his ideas
G used and emphasized experiment – Galileo sought to thoroughly test his ideas where A did not
Wanted to know the “how” of things rather than the “why”

7. 2.2 – Galileo’s Concept of Inertia - Continued
Leaning Tower of Pisa Experiment –
Showed that except for air friction, objects of different weights fall to the ground at the same time
This experiment disproved Aristotle’s idea that heavier objects fall faster than lighter objects.

9. 2.2 – Galileo’s Concept of Inertia - Continued
Another gigantic discovery!
Aristotle was wrong about forces being required to keep objects moving
Galileo said that a force is required to start an object moving.
But no force is required to keep it moving
Another force, friction, is slowing the movement
WITH OUT friction, an object does not require a force to keep it moving

10. Friction

11. 2.2 – Galileo’s Concept of Inertia - Continued
G’s rolling ball experiment illustrates his ideas – p. 18 Fig. 2.3

12. 2.2 – Galileo’s Concept of Inertia - Continued
Still, both A and G left one concept out – Inertia!
All objects show the same property of motion
Inertia - The tendency of things to remain as they are
Objects in motion tend to stay in motion
Objects at rest tend to stay at rest

13. Inertia

14. 2.3 - Galileo, Speed, and Velocity
What is speed?
Speed is the distance covered per unit of time
speed = distance / time
s = d / t
SI units of speed - m/s and km/hr are the most common

15. 2.3 - Galileo, Speed, and Velocity
What is velocity?
Velocity is speed with a direction
speed = how fast
velocity = how fast AND in what direction

16. 2.3 - Galileo, Speed, and Velocity
Velocity is a vector quantity.
A vector has both magnitude (how much) and a direction (which way)
Vectors are commonly used when studying the laws of motion.

17. 2.3 - Galileo, Speed, and Velocity
If something has a constant speed - the speed does not change.
It does not go up or down
If something has a constant velocity - the velocity does not change.
It has a constant speed and a constant direction
Neither changes, therefore it is moving in a straight line.

18. Sources
Friction Image -
Inertia Animation -