Chapter 4 Newton’s First Law: The Law of Inertia.

Chapter 4Chapter 4

Newton’s First Law: The Law of InertiaNewton’s First Law: The Law of Inertia

1.1. ARISTOTLE ON MOTIONARISTOTLE ON MOTION

Aristotle attempted to Aristotle attempted to understand motion by understand motion by

classificationclassification..

Two Classes:Two Classes:

Natural and ViolentNatural and Violent

Ancient Greek philosopher and scientist Aristotle developed the earliest theory of how things move.

natural motion – motion that could maintain itself without theaid of an outside agent. (pushing a rock off theledge and it falls to the ground, liquids falling or running downhill, air rising, flames leaping

upward)Aristotle believed everything was made of four elements

Fire

Air

Water

Earth

Aristotle'sPeriodicTable

“Natural Motion”(vertical)

“Violent Motion” (horizontal)

Aristotelian UniverseTerrestrial Realm

Earth and Water tend to sink

Composition predicts motionnatural tendencies

Overall tendency to seek rest Objects are corruptible (changing)

Fire and Air tend to rise

How things Move

Fire

Air

Water

Earth

Aristotle'sPeriodicTable

“Natural Motion”(vertical)

“Violent Motion” (horizontal)

• Each element has its own natural motion, and its own place that it strives to be.

• Aristotle believed an object’s natural motion was determined by how much of each element the object contained (rocks sink in water because they contain mostly earth, wood floats because it contains mostly air)earth moves downward

because Earth’s centeris its natural resting place

water’s naturalresting placeis on top of earth

• Violent Motion – motion that forced objects to behave contrary to an objects natural motion, meaning an external push or pull was needed

Aristotle’s Laws of Motion –Summary

1. Natural Motions are determined by the innate “desire” of the moving object to returnto it’s “natural resting place “. The proper state was “at rest”.

2. Violent Motions occur because outside influences give motion to the object. As soon asforce “runs out”, the object stops. This was the belief for 2000 years.

3. Objects composed of ether travel in gigantic, perfect circles centered on the Earth which was assumed to be at rest.

Stationary Earth• Earth is heavy.

– Easier to imagine the sky can move

• It looks like the sky is moving.– We have no sensation of our motion.

• A rotating Earth would make objects fly off of the surface.

• It would take a huge force to move the Earth so the Earth must not be moving.

• The Earth must be the center of the universe.

Nicholas Copernicus1473 - 1543

(Niklas Koppernigk)

2. Nicholas Copernicus developed a mathematical model for a Sun-centered

solar system based on astronomical observations.

Galileo Galilei1564-1642

• He was among the first to turn a telescope to the sky.

• He developed the Scientific Method.• He believed in the popularization of science.• He defined inertia as resistance to change in

motion.

3. Galileo Galilei helped overthrow Aristotelian physics. Because of hisradical ideas he was confined to house arrest for the last 10 years of his life.

Galileo argued that only when friction is present is a force needed to keep an object in motion. Friction is a force caused by irregularities in the surfaces of objects in contact. Friction acts in a way which opposes motion.

I do not feel obliged to believe that the same God who has endowed us with sense, reason, and intellect has intended us to forgo their use. -Galileo

As a ball rolls down an incline its speed increases. Push the ball up incline, it slows down, stops and rollsback down. Suppose the incline is nearly horizontal. Obviously, due to friction the ball would probably come to a stop. Galileo understood that this slowing is due to the roughness of the incline and ball. Let’stry to visualize the experiment and neglect friction. The ball would then speed up on any downward slopeand slow down on any upward slope. Let’s look at the limiting case when the incline is perfectly horizontal.In the absence of friction, a ball that got started rolling horizontally would roll forever. This is contradictory to Aristotle’s view of motion

Aristotle

What type of motion? All objects resist a change in their motion so what are the outside influences that cause motion?

Celestial Natural Violent

Ether

Circles What is How muchit made of motion given?

Fire Air Water Earth

Natural Resting Place?

Galileo

Isaac Newton1642 - 1727

Newton’s Laws

Every object continues to move as it has been moving unless acted upon by an external force.

The 1st Law

Law of Inertia – An object will move in a straight line at a constant speedUNLESS acted upon by an external force

- objects in motion stay in motion- objects at rest stay at restUNLESS influenced in some way

YOU

PASSENGER

Car begins turning here

you start turning here

•outside influence – tires on carfriction on tires

•outside influence – door on you

Another example: “Canis Insanis”

you fastened to you seat belt

Miss Pickles standing on the console

Jeep

apply brakes atred light

you stop too!

trip to thevet needed

Mass and Inertia• Inertia is a property of a body that resists

changes in motion.

• Mass is a measure of the amount of matter in a body.

• Inertia and Mass are the same concept.

– Units – Kg or slugs

• Mass and weight are NOT the same concept but they are proportional.

• Mass and volume are NOT the same concept but they are proportional.

Weight• Force of gravity pulling on the mass of

the body.– Units – Newtons or Pounds

• A newton is about the weight of a small apple.– One kg weighs 2.2 lbs.

• W = mg– g = acceleration due to gravity at the

surface of the Earth• = 32 ft/s2 = 9.8 m/s2

6. Net Force

• A force is a push or a pull. • Force is a vector

– We must add all the forces acting on the body

5 N5 N 10 N

5 N5 N 0 N

5 N 10 N 5 N

7. Static Equilibrium7. Static EquilibriumVelocity is zero

Examples:

Hanging from a tree

Weighing yourself on a set of scales

Computer setting on a table

Car parked on an incline

Normal up

Weight down

Weight down

Weight downWeight down

Scales pushing up

Tree pulling up

NormalFriction

Dynamic Equilibrium• When the net force

is zero (F = 0). This refers to all of the forces acting on an object, including friction. Acceleration is

zero.Velocity may not

be zero.

How things Move• Gravity is the “outside influence” that causes the ball to drop

• In fact all falling objects fall at the same rate, called the acceleration of gravity (neglecting air resistance)

• Drop different objects their speed will increase at the same rate!

• Their speed will increase by ~ 10 m/s (32 ft/s) every second

Free Fall and FrictionAir friction slows the

ping pong ball down

compared to the

billiard ball.

In the absence of air,

Galileo predicted

that they would fall

at the same rate, like

a wooden and iron

ball.

How things Move

Two things Galileo figured out as a result of the law of inertia

1. Assuming that the only “outside influence” has the same effect on all objects, all should fall at the same rate. [Galileo’s Law Falling]

Galileo knew not of force or the law of gravity, these conceptscame along later. Rather he took this “same effect” idea from hisobservations and idealizations

2. There is an outside influence called friction present in the “real world”. Friction acts differently on different objects.

Galileo’s reasoning for “why moving things stop”

Uniform Circular Motion – acceleration by virtue of a change in direction“centripetal acceleration” – center pointing

Imagine that a car travels with a constant speed around a circular track

In this type of motion, theacceleration is alwaysdirected toward the centerof the circular path

R

C

instantaneous velocity atthis point on the track

where the carends up a momentlater if thereis no acceleration

the difference in velocitypoints in the direction ofthe acceleration…….toward the center

instantaneous velocity of thecar at a new point on thetrack a moment later….samespeed – different direction

Quantitative:travel at speed = saround circle of radius = Ryour “centripetal” acceleration is

rs

ac

2

C = center of the circular pathR = radius of the circular path

• Motion not on earth followed a different set of rules. Motion in the heavens had no beginning or end. Circular paths were considered natural for the starsand the planets.

• 5th element – ether (from the Greek word for to kindle or blaze) – had no weight and was unchangeable, and perfect in every way

• Moon, sun, planets and stars were made of ethercelestialmotion

“perfect circles”

• Ether’s natural place was in the “heavens” and it moved in perfect circles.

• Objects on earth could not move the way the stars did because they did not contain ether.

• Aristotle's physics governed science until about the mid 16th century.

• It was popular because it reinforced •religious beliefs.