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

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.

2.1 – Aristotle’s Classification of Motion

Aristotle believed that there are two types of motion -

Natural Motion

Unnatural Motion

2.1 – Aristotle’s Classification of Motion -

ContinuedNatural 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

2.1 – Aristotle’s Classification of Motion –

ContinuedUnnatural motion – requires forces

Such as forces imposed by animals or people

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”

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.

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

Friction

2.2 – Galileo’s Concept of Inertia - Continued

G’s rolling ball experiment illustrates his ideas – p. 18 Fig. 2.3

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

Inertia

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

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

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.

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.

Sources

• Friction Image - http://www.physicsphenomena.com/InclinedPlanegeneric.jpg

• Inertia Animation - http://teachertech.rice.edu/Participants/louviere/Newton/law1.html