Chapter 2

Newton’s 1st Law of Motion – Inertia

Newton’s 1st Law of Motion

Aristotle on MotionCopernicus and the Moving EarthGalileo and the Leaning TowerGalileo’s Inclined PlaneNewton’s 1st Law of MotionNet ForceThe Equilibrium RuleSupport ForceEquilibrium of Moving ThingsThe Moving Earth

Aristotle on MotionAristotle (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 harderObject 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:• 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 MotionNatural MotionStraight up or down motion – earthly objectsCircular motion – celestial objects

Different rules than for terrestrial objectsPerfect spheresMade of perfect unchanging substance (quintessence)

Violent Motion Resulting from pushing or pulling forcesExternally 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.

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.”

“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 PlaneGalileo 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 PlaneGalileo - 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 surfacesWavelength of light4000-8000 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.

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 MotionExamples:

Tablecloth experiment (book and paper);Plastic bags on a roll at the grocery store;Dry ice puckNASA payload simulator

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.

Multiple Forces

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.

Net Force

The Equilibrium RuleIn equilibrium there is no net acceleration and the velocity remains constant.

The Net Force is zero

Vector versus scalar discussion.

Ques: Vector – examplesVelocityAccelerationDisplacement

Scalar - examplesDistance, Speed, MassTemperature

Ques: Vector – examplesVelocityAccelerationDisplacement

Scalar - examplesDistance, Speed, MassTemperature

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 ForceAlso 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.”

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.

Support Force

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 FrictionQuestions:

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?

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.

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