Newton’s First Law - InertiaNewton’s First Law - Inertia

Chapter FourChapter Four

Aristotle on MotionAristotle on Motion

Greek scientist – 400 BCGreek scientist – 400 BC

Two types of MotionTwo types of Motion– Natural – Straight-up motion (smoke) or Natural – Straight-up motion (smoke) or

straight-down motion (falling) or circular straight-down motion (falling) or circular (planets)(planets)

Objects would seek their “natural” resting Objects would seek their “natural” resting placesplaces

Not thought to be caused by forcesNot thought to be caused by forces

Aristotle on MotionAristotle on Motion– Violent – imposed Violent – imposed

motionmotionFrom push or pullFrom push or pull

External causesExternal causes

Examples – cart, tug-of-Examples – cart, tug-of-warwar

Objects in natural resting Objects in natural resting positions would not move positions would not move by themselvesby themselves

Copernicus and Moving EarthCopernicus and Moving EarthAstronomer – 1473-1543Astronomer – 1473-1543

Assumed earth and Assumed earth and planets moved around the planets moved around the sunsun

Used to think earth was Used to think earth was the center of the universethe center of the universe

Published his findings Published his findings around the time of his around the time of his deathdeath

Galileo on MotionGalileo on MotionBelieved Believed CopernicusCopernicusWas put under Was put under house arrest by house arrest by the churchthe churchBiggest Biggest contribution – contribution – force in NOT force in NOT needed to keep an needed to keep an object movingobject moving

Force and FrictionForce and Friction

Force – push or pullForce – push or pull

Friction – force Friction – force between materials that between materials that touchtouch– Caused by surface Caused by surface

irregularitiesirregularities– Friction stops motionFriction stops motion

Galileo’s ExperimentsGalileo’s Experiments

Only when friction is present is a Only when friction is present is a force needed to get something force needed to get something movingmoving

Tested his idea with rolling balls on Tested his idea with rolling balls on plane surfaces at different angles.plane surfaces at different angles.

Rolling down a hill,the speed increases

Galileo’s experimentsGalileo’s experiments

Rolling up a hill,speed

decreases

No slope,speed

remainsconstant

He reasoned the without friction – the ball would continue forever

Galileo’s experimentsGalileo’s experiments

The balls would rise up on each side the same height

So what would happen if it was flat?

The ball would go on forever….

InertiaInertiaA moving body A moving body keeps movingkeeps moving

Galileo Galileo – Concerned with Concerned with

how it worked – how it worked – not whynot why

– Experiments Experiments (not logic) (not logic) leads to leads to knowledgeknowledge

Sir Isaac NewtonSir Isaac Newton

Born Christmas day – in the same Born Christmas day – in the same year as Galileo’s deathyear as Galileo’s death

Had made his famous laws by the Had made his famous laws by the time he was 24.time he was 24.

11stst law of Motion - Inertia law of Motion - Inertia

Every object continues in a state of Every object continues in a state of rest, or of motion in a straight line at rest, or of motion in a straight line at constant speed, unless it is constant speed, unless it is compelled to change by forces compelled to change by forces exerted on itexerted on it

Examples – coin on paper, dishes on Examples – coin on paper, dishes on table, hockey puck on air table, table, hockey puck on air table, Pioneer and VoyagerPioneer and Voyager

QuestionQuestion If the force of gravity If the force of gravity between the sun and between the sun and planets suddenly planets suddenly disappeared, what type of disappeared, what type of path would the planets path would the planets follow?follow?

Is it correct to say that the Is it correct to say that the reason an object resists reason an object resists change and persist in its change and persist in its state of motion is that it state of motion is that it has inertia?has inertia?

AnswerAnswerThe planets would move in straight lines at The planets would move in straight lines at constant speed (velocity)constant speed (velocity)

No, we don’t know the reason why objects No, we don’t know the reason why objects persist in their motion when nothing acts persist in their motion when nothing acts on them, but we call this property inertia. on them, but we call this property inertia. We understand many things – but some We understand many things – but some things – gravity and inertia for example, things – gravity and inertia for example, are still unexplained.are still unexplained.

InertiaInertia

Mass – a Measure of InertiaMass – a Measure of Inertia

Kicking a can Kicking a can – EmptyEmpty– Filled with toothpicksFilled with toothpicks– Filled with nailsFilled with nails

Amount of inertia is Amount of inertia is related to mass of related to mass of objectobject

Mass is NOT volumeMass is NOT volume

Volume – is spaceVolume – is space– Cubic centimeters, cubic meters, litersCubic centimeters, cubic meters, liters

Which weighs more Which weighs more – 1 kg of feathers1 kg of feathers– 1 kg of nails?1 kg of nails?

Which has more volume?Which has more volume?

Mass is not WeightMass is not Weight

Mass – amount of matter, depends Mass – amount of matter, depends on number and kind of atomson number and kind of atoms

Weight – force from gravity acting on Weight – force from gravity acting on itit

They are proportional to each other – They are proportional to each other – but not equal to each otherbut not equal to each other

QuestionQuestion

Does a 2 Kg iron block have twice as Does a 2 Kg iron block have twice as much inertia as a 1 kg block of iron?much inertia as a 1 kg block of iron?

Twice as much mass?Twice as much mass?

Twice as much volume?Twice as much volume?

Twice as much weight (in the same Twice as much weight (in the same location?)location?)

AnswerAnswer

Yes to all questions. It has twice as Yes to all questions. It has twice as many atoms, so amount of matter, many atoms, so amount of matter, mass, weight will all increase. mass, weight will all increase.

QuestionQuestion

Does a 2-kg bunch of bananas have Does a 2-kg bunch of bananas have twice as much inertia as a 1 kg loaf twice as much inertia as a 1 kg loaf of bread?of bread?

Twice as much mass?Twice as much mass?

Twice a much volume?Twice a much volume?

Twice as much weight (when Twice as much weight (when weighed in same location)weighed in same location)

AnswerAnswer

Two kilograms of anything will have Two kilograms of anything will have twice the mass and inertia. In the twice the mass and inertia. In the same location, 2kg will have twice same location, 2kg will have twice the weight as 1kg. However, volume the weight as 1kg. However, volume and mass are related by density. and mass are related by density. Bananas are much more dense than Bananas are much more dense than bread, therefore they will occupy less bread, therefore they will occupy less space.space.

WeightWeight

In USA – measured in poundsIn USA – measured in pounds

In World – measured in In World – measured in kilogramskilograms

1 Kg (mass) = 2.2 pounds 1 Kg (mass) = 2.2 pounds (weight)(weight)

How many kilograms do you How many kilograms do you weigh?weigh?– Your weight x 1 kg/2.2 pound =Your weight x 1 kg/2.2 pound =

ForceForce

A push or a pullA push or a pull

Measured in NewtonsMeasured in Newtons

1 N is about the weight of a cooked 1 N is about the weight of a cooked burgerburger

1 kg weighs 9.8 N1 kg weighs 9.8 N– 1 kg = 9.8 N = 2.2 lbs (on earth)1 kg = 9.8 N = 2.2 lbs (on earth)

Weight = mass X acceleration Weight = mass X acceleration F = ma or F = mg (gravity)F = ma or F = mg (gravity)

QuestionQuestion

1 kg of nails weighs 9.8 N at Earth’s 1 kg of nails weighs 9.8 N at Earth’s surface. Does 1 kg of yogurt also surface. Does 1 kg of yogurt also weigh 9.8 N at Earth’s surface?weigh 9.8 N at Earth’s surface?

AnswerAnswer

Yes, anything that masses 1 kg will Yes, anything that masses 1 kg will have a weight of 9.8 N.have a weight of 9.8 N.

Net ForceNet Force

According to 1According to 1stst law – object either law – object either stay at rest or stay moving – unless a stay at rest or stay moving – unless a force changes itforce changes it

5 N 5 N

Is this block at rest?

Net ForceNet Force

Combination of all forces acting on Combination of all forces acting on an object an object – If the net force is equal to zero then If the net force is equal to zero then

there is NO motionthere is NO motion

5 N 5 N

Net Force CombinationNet Force Combination

The two applied forces work together The two applied forces work together to make a net force of 10 Nto make a net force of 10 N

5 N5 N

10 N

Equilibrium Equilibrium

When net forces equal zeroWhen net forces equal zero

Example-Forces on a book on a tableExample-Forces on a book on a table– GravityGravity– Support forceSupport force from the table from the table

Also called Also called normal forcenormal force

Caused by the atoms in the table “pushing” Caused by the atoms in the table “pushing” back back

The book compresses the atoms (like a The book compresses the atoms (like a spring)spring)

– The table pushed back with the same The table pushed back with the same force that gravity pulls downforce that gravity pulls down

EquilibriumEquilibrium

Example – Hanging from a ropeExample – Hanging from a rope– Atoms are not compressed – they are Atoms are not compressed – they are

stretchedstretched– The force of tension = force of weightThe force of tension = force of weight

What if hanging by two ropes?What if hanging by two ropes?– The tension is divided equally by the The tension is divided equally by the

ropesropes

QuestionQuestion

When you step on a bathroom scale, When you step on a bathroom scale, the downward force by your feet and the downward force by your feet and the upward force supplied by the the upward force supplied by the floor compress a calibrated spring. floor compress a calibrated spring. In effect, the scale measures the In effect, the scale measures the floors support force. What will the floors support force. What will the scale read if you stand on 2 scales?scale read if you stand on 2 scales?

AnswerAnswer

The scales will add up to your weight. The scales will add up to your weight. If you balance your weight, the If you balance your weight, the scales will read the same. If you shift scales will read the same. If you shift your weight to one side, the scales your weight to one side, the scales will have different weights, but will will have different weights, but will still add up to your entire weight.still add up to your entire weight.

Using force scalesUsing force scalesThe scales The scales are in are in equilibriumequilibrium

The The resultant of resultant of the forces the forces equal the equal the downward downward pullpull10 N10 N

10 N5 N 5 N

Vector Addition of ForcesVector Addition of Forces

Non-vertical orientation of forcesNon-vertical orientation of forces– Tension is greaterTension is greater– Depend on angle from the verticalDepend on angle from the vertical

Use the same vector adding to Use the same vector adding to calculate resultant forcescalculate resultant forces

Spring scales at an AngleSpring scales at an Angle

10 N

10 N60 60

ClotheslineClothesline

Why can a clothesline support you Why can a clothesline support you vertically but not horizontally?vertically but not horizontally?

50 N

50 N50 N

50 N

ClotheslineClotheslineUse the Use the parallelogram parallelogram method to find the method to find the forceforce

50 N

50 N

Resultant

•Note - as the rope becomes more horizontal resultant force would become close to infinity!!

The Moving EarthThe Moving Earth

When Copernicus first announced the When Copernicus first announced the idea that the Earth moved around idea that the Earth moved around the sun at 30 km/sec, people were the sun at 30 km/sec, people were skepticalskeptical

Reason – How can a bird jump down Reason – How can a bird jump down out of a tree and catch a worm if the out of a tree and catch a worm if the earth moves 30 km/sec while the earth moves 30 km/sec while the bird is flying?bird is flying?

RefuteRefute

The whole environment is moving at The whole environment is moving at 30 km/sec30 km/sec– TreeTree -Bird-Bird– WormWorm -Air-Air

400 years ago people had trouble 400 years ago people had trouble with high speeds with high speeds

Today we know the coins flip the Today we know the coins flip the same in a car or on the groundsame in a car or on the ground

ConclusionConclusion

Aristotle – didn’t recognize inertia Aristotle – didn’t recognize inertia because he thought there where because he thought there where different rules for heaven and earthdifferent rules for heaven and earth

Galileo and Newton – Recognized Galileo and Newton – Recognized that all moving things follow the that all moving things follow the same rulessame rules– Things keep on moving unless a force Things keep on moving unless a force

(friction) stops it(friction) stops it