newton’s first law - inertia
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Newton’s First Law - Inertia. Chapter Four. Aristotle on Motion. Greek scientist – 400 BC Two types of Motion Natural – Straight-up motion (smoke) or straight-down motion (falling) or circular (planets) Objects would seek their “natural” resting places Not thought to be caused by forces. - PowerPoint PPT PresentationTRANSCRIPT
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