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Chapter 2
Newton’s 1st Law of Motion –
Inertia
Newton’s 1st Law of Motion
Aristotle on Motion
Copernicus and the Moving Earth
Galileo and the Leaning Tower
Galileo’s Inclined Plane
Newton’s 1st Law of Motion
Net Force
The Equilibrium Rule
Support Force
Equilibrium of Moving Things
The Moving Earth
Aristotle on Motion
Aristotle (384-322 B.C.) Greek philosopher, scientist, &
educator
Earth, Air, Fire & Water – Everything was a combination of
these
Every object has a “proper” place.
Every object will strive to reach this proper place.
Heavier object would strive harder
Object should fall at a rate proportional to their weight
Heavy objects fall faster
(Observationally this is TRUE)
Aristotle
Aristotle:
• Astute observer
• Studied problems
around him.
• Motion involved
resistive medium.
• A vacuum
impossible
• Objects require a
push or pull to
remain in motion.
Aristotle’s Universe
Types of Motion
Natural Motion
Straight up or down motion – earthly objects
Circular motion – celestial objects
 Different rules than for terrestrial objects
 Perfect spheres
 Made of perfect unchanging substance (quintessence)
Violent Motion
Resulting from pushing or pulling forces
Externally caused
At the beginning of the 16th
century it was thought:
1. Earth must be in its proper
place.
2. A force capable of moving
the earth is inconceivable.
3. Therefore the earth does
not move.
In Summary
1. All motion is due to:
the nature of the object (natural)
or a sustained push or pull (violent)
2. An object in its proper place will not move unless
forced
3. Except for celestial objects the normal state is
one of rest.
4. Heavy objects fall faster
Copernicus and the Moving Earth
Copernicus (1474-1543), Polish astronomer
Viewed from earth the orbits of the other
plants sometimes exhibited a retrograde type
of motion.
http://alpha.lasalle.edu/~smithsc/Astronomy/retrograd.html
“In its final form, the model (earth centric) was extremely complicated, requiring
many nested levels of epicycles, and with even the major orbits offset so that they
were no longer truly centered on the Earth.
Despite all of this fine tuning, there remained significant discrepancies between the
actual positions of the planets and those predicted by the model.
Nevertheless, it was the most accurate model available, and it remained the
accepted theory for over 13 centuries, before it was finally replaced by the model
of Copernicus.”
Copernicus and the Moving Earth
Simplest way to account for the motion of the Sun, Moon and
planets was to assume that the earth was moving.
In particular Copernicus assumed that the earth and the other
planets all revolved around the Sun.
Copernicus and the Moving Earth
Aristotle’s teachings had been embraced by the Church
Finding fault with Aristotle was equivalent to finding fault with
the teachings of the Church.
“For many Church leaders, the idea of a moving Earth threatened
not only their authority but the very foundations of faith and
civilization as well.”
Copernicus’ research was published just before his death in 1543.
Galileo and the Leaning Tower
Galileo Galilei (1564 – 1642)
Discredited Aristotle’s ideas on motion through
experimentation.
The Leaning Tower experiments demolished
Aristotle’s falling body hypothesis.
Of, course not everyone was convinced!
Galileo’s Inclined Plane
Galileo was concerned with HOW things moved - not the WHY
Inclined planes reduced the acceleration of objects and made it
easier to study them.
Galileo’s Inclined Plane
Galileo - In the absence of friction or other opposing forces, a
horizontally moving object will continue moving indefinitely.
The two inclined planes together showed this in another way.
• As the angle of the second incline decreased the object
rose just as high but traveled a longer distance.
• At a zero angle the object would travel forever.
Inertia is the property of objects that causes them to resist
changes in their motion.
Galileo’s Inclined Plane
Aristotle missed inertia because he didn’t envision
motion without friction. Resistance to motion was built
into Aristotle’s theory.
Galileo overturned Aristotle with Inertia. That
property which resists change enabled one of the
biggest changes ever.
FRICTION
Reflecting surfaces
Wavelength of light
4000-8000 Angstroms (10-10m)
1 Angstrom = 0.1 microns (10-6m)
100 microns = Diam of human hair
If the surface variations are less than
the wavelength of light it will reflect
the image undistorted. It will act as a
mirror.
The Importance of Friction
The Roller Coaster Story
• A roller coaster is pulled by a chain to the highest point
in its run.
• From there gravity takes over.
• Friction tends to keep the speed down. It is possible for
the cars to go too fast.
• A wet track has less friction.
• After a shower the cars are run empty and timed.
• If the cars come back too soon the speed is too fast and
the track needs to dry some more.
Newton’s 1st Law of Motion
Isaac Newton (1642 – 1727)
Newton took Galileo’s principle of inertia and built it into his
First Law of Motion (also known as the Law of Inertia).
“Every object continues in its state of rest, or of uniform
motion in a straight line, unless it is compelled to change
that state by forces impressed upon it.”
From Newton’s Principia, translated from Latin
“At rest” is just a state where the velocity is zero.
Newton’s 1st Law of Motion
Examples:
Tablecloth experiment (book and paper);
Plastic bags on a roll at the grocery store;
Dry ice puck
NASA payload simulator
Multiple Forces
Changes in motion are caused by the application of
forces.
(Forces give rise to changes in velocity, i.e.
accelerations)
Many sources: Push, Pull, Hit with a bat, Gravity,
Electrical, Magnetic.
Net Force
If more than one force acts on an object we must take into
consideration all of these forces.
The “sum” of all these forces is called the Net Force
Example: A block of weight W on a frictionless surface. Strings
with weights w1 and w2 attached to each end and hung over
massless, frictionless pulleys. Discuss the motion of W as the
difference between w1 and w2 is changed.
The Equilibrium Rule
In equilibrium there is no net acceleration and the
velocity remains constant.
The Net Force is zero
Vector versus scalar discussion.
Ques:
Vector – examples
Velocity
Acceleration
Displacement
Scalar - examples
Distance, Speed, Mass
Temperature
What is the Weight of the Plank?
What is the Tension in the Cable?
Equilibrium
Tension in the two cables changes continuously as
the painter walks across the plank.
Support Force
Also called the Normal force
The direction perpendicular to a surface is called the normal
direction.
This is one of those words that physicists and mathematicians
have given a special meaning that is different from the
meaning found in the general population.
Another example: Something is “generally” true.
Physicst – “It is true all the time.”
John Doe – “It is true most of the time, but not
always.”
Support Force
Within fairly wide limits it provides whatever magnitude of
force is needed to keep the object in vertical equilibrium. Up to
its strength of material limit.
Flat surface – supplies a normal force to counteract gravity.
Equilibrium of Moving Things
Forces balance  Σ F = 0
No speeding up, no slowing down. The girl is
supplying energy and doing work against friction.
Static & Dynamic Friction
Questions:
1. Is the friction force dependent on the velocity of the
block? Does it increase as the velocity increases?
2. A bowling ball is in equilibrium when at rest. If the ball is
moving at a constant speed is it still in equilibrium?
3. If it takes 150 N of force to make a box travel at a
CONSTANT velocity what is the magnitude of the force of
friction?
4. If the maximum friction force is 75N and the girl applies
150N to the box what is the net result?
Static & Dynamic Friction
Explain static (no motion) and dynamic (moving) friction
The Moving Earth
Aristotle didn’t recognize the
idea of inertia because he did not
see that all moving things follow
the same rules.
Vertical motion was natural.
Horizontal motion was unnatural.
Review – Pulleys and Tension