Newton’s First Law of Newton’s First Law of Motion—Inertia Motion—Inertia

Chapter 4Chapter 4

Aristotle (4Aristotle (4thth Century B.C.) Century B.C.)

Believed in two different types of motion: Believed in two different types of motion: natural motionnatural motion & & violent motionviolent motion

Natural MotionNatural Motion – either straight up or – either straight up or straight down, objects would seek out their straight down, objects would seek out their natural resting places (it was “natural” for natural resting places (it was “natural” for heavy things to fall and light things to rise)heavy things to fall and light things to rise)

Violent MotionViolent Motion – imposed motion, result of – imposed motion, result of forces that pushed or pulledforces that pushed or pulled

The proper state of objects was that of The proper state of objects was that of rest.rest.

AristotleAristotle

Copernicus (1473-1543 A.D.)Copernicus (1473-1543 A.D.)

Because Earth was already in its natural Because Earth was already in its natural resting place, it could not move (there was resting place, it could not move (there was no force big enough to move Earth)no force big enough to move Earth)

Copernicus concluded that the only way to Copernicus concluded that the only way to make sense of the way the planets move make sense of the way the planets move is to assume that Earth and the other is to assume that Earth and the other planets move around the sunplanets move around the sun

He worked in secret to avoid being He worked in secret to avoid being persecuted by the churchpersecuted by the church

CopernicusCopernicus

GalileoGalileo The foremost scientist of the late-The foremost scientist of the late-

Renaissance in ItalyRenaissance in Italy Outspoken in his support of Copernicus, Outspoken in his support of Copernicus,

later put on trial and confined to house later put on trial and confined to house arrestarrest

A A forceforce is any push or pull is any push or pull Friction – Friction – the force that acts between the force that acts between

materials that touch as they move past materials that touch as they move past each othereach other

If friction were absent, an object would If friction were absent, an object would need no force to remain in motionneed no force to remain in motion

GalileoGalileo

Galileo’s Inclined PlanesGalileo’s Inclined Planes

Noted that a ball that rolls down an Noted that a ball that rolls down an inclined plane picks up speedinclined plane picks up speed

When the same ball rolls up the inclined When the same ball rolls up the inclined plane, it slows downplane, it slows down

When a ball rolls on a flat horizontal When a ball rolls on a flat horizontal surface, it rolls at near constant velocitysurface, it rolls at near constant velocity

Stated that with the absence of friction, the Stated that with the absence of friction, the ball would roll foreverball would roll forever

Galileo’s Inclined Planes Cont.Galileo’s Inclined Planes Cont.

If you have two inclined planes face each If you have two inclined planes face each other, a ball rolled down one plane will other, a ball rolled down one plane will reach nearly the same height as it rolls up reach nearly the same height as it rolls up the other planethe other plane

He noticed that the ball ended up at the He noticed that the ball ended up at the same height, even is the plane was same height, even is the plane was elongated or at a different angleelongated or at a different angle

Inertia – Inertia – the property of a body to resist the property of a body to resist change (the tendency of a moving body to change (the tendency of a moving body to keep moving and every material object keep moving and every material object resists change to its state of motion)resists change to its state of motion)

Inclined PlanesInclined Planes

Newton’s First LawNewton’s First Law

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

An object will keep doing what it’s already An object will keep doing what it’s already doingdoing

Toss an object out the door of the Toss an object out the door of the International Space Station, and it will International Space Station, and it will keep moving at the speed you threw it at keep moving at the speed you threw it at forever (no friction in space)forever (no friction in space)

Newton’s 1Newton’s 1stst Law Law

The Law of InertiaThe Law of Inertia

The Law of InertiaThe Law of Inertia

Pioneer and VoyagerPioneer and Voyager

After they initially After they initially were sent into space, were sent into space, these two spacecraft these two spacecraft utilize Newton’s Law utilize Newton’s Law of Inertia to move of Inertia to move through spacethrough space

They are now cruising They are now cruising outside of our solar outside of our solar systemsystem

Mass – A Measure of InertiaMass – A Measure of Inertia The amount of inertia an object has depends on its The amount of inertia an object has depends on its

mass—which is roughly the amount of material mass—which is roughly the amount of material present in the objectpresent in the object

Mass is NOT volume, the measure of space that an Mass is NOT volume, the measure of space that an object takes upobject takes up

Mass is NOT weight, the force of gravity on an objectMass is NOT weight, the force of gravity on an object Mass is a measure of the inertia that an object exhibits Mass is a measure of the inertia that an object exhibits

in response to any effort made to start it, stop it, or in response to any effort made to start it, stop it, or otherwise change its state of motionotherwise change its state of motion

Mass and weight may not be the same, but they are Mass and weight may not be the same, but they are proportional to each otherproportional to each other

Weight = mass x acceleration due to gravityWeight = mass x acceleration due to gravityMeasured in Newtons (N)Measured in Newtons (N)

A 1-kg bag of nails weighs 9.8 N on the surface of Earth A 1-kg bag of nails weighs 9.8 N on the surface of Earth (2.2 lbs.)(2.2 lbs.)

Net ForceNet Force

In the absence of a In the absence of a net forcenet force, objects do , objects do not change their state of motionnot change their state of motion

If you push with equal and opposite forces If you push with equal and opposite forces on opposite sides of an object at rest, it on opposite sides of an object at rest, it will remain at restwill remain at rest

Net ForceNet Force – the combination of all forces – the combination of all forces acting on an objectacting on an object

We use force diagrams to figure out the We use force diagrams to figure out the net forcenet force

Force DiagramsForce Diagrams

EquilibriumEquilibrium If only the force of gravity was acting on an If only the force of gravity was acting on an

object at rest, the object would constantly object at rest, the object would constantly be in free fallbe in free fall

The fact that the object is at rest means The fact that the object is at rest means that another force must be acting upon itthat another force must be acting upon it

The other force exactly balances out the The other force exactly balances out the weight of the object and produces a net weight of the object and produces a net force of zeroforce of zero

The other force is called the The other force is called the support support forceforce, or , or normal forcenormal force

Equilibrium Equilibrium – the net force on an object is – the net force on an object is zerozero

EquilibriumEquilibrium

Vector Addition of ForcesVector Addition of Forces We can use the same vector addition We can use the same vector addition

techniques on forces as we did on techniques on forces as we did on velocities!velocities!

Force, like velocity has a magnitude and a Force, like velocity has a magnitude and a directiondirection

For any pair of scales, ropes, or wires For any pair of scales, ropes, or wires supporting a load, the greater the angle supporting a load, the greater the angle from the vertical, the larger the tension from the vertical, the larger the tension force in themforce in them

The resultant of the tension forces in a The resultant of the tension forces in a rope must be equal and opposite to the rope must be equal and opposite to the load being supportedload being supported

Vector Addition of ForcesVector Addition of Forces

Assignment (Due Wednesday 10/7)Assignment (Due Wednesday 10/7)

Read Chapter 4 (pg. 43-55)Read Chapter 4 (pg. 43-55) Do Ch. Assessment #21-39 (pg. 57-58)Do Ch. Assessment #21-39 (pg. 57-58) Appendix F #1-10 (pg. 665-666)Appendix F #1-10 (pg. 665-666)