Transcript Chapter 2

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