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

2.1 – Aristotle’s Classification of Motion – Continued Unnatural 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