.

Or …

Things don’t just happen all by themselves,

there’s a reason.

350 BC - Aristotle350 BC - Aristotle

• Aristotle divided motion into 2 types :

• Natural Motion

• Objects seek their

natural resting positions

• Violent or imposed motion

Natural Motion• Mostly considered straight up

• Or

Straight down … like a boulder falling or smoke drifting up in the air like clouds or the circular motions of the planets

• These motions were considered “natural” and not caused by forces.

VIOLENT MOTION

• Imposed Motion

• Motion with an external cause

• The Result of forces that push or pull.

• EXAMPLES :

• A cart moving a rope tugged

a ship pushed by the force of the wind

Natural and Violent Motion

• For almost 2000 years it was considered that if an object was moving “against its nature”, then a force of some kind was involved.

• If there were no force then there would be no motion (except vertically : falling rocks and rising smoke).

• Therefore:

• The proper state of objects is at rest.

Nikolai Kopernik (Copernicus)

• Nicolaus Copernicus – 1473 – 1543• The Theory of the Moving Earth • The Earth was not the center of the Universe• The Earth and planets revolved around the Sun

(contrary to religious orthodoxy)

• Working in secret, Copernicus’ work was not published until the day of his death.

• Copernicus needed a champion for his ideas and he found Galileo.

THE UNIVERSEIS HELIOCENTRIC, NOT GEOCENTRIC !

Heliocentric vs. Geocentric

• Heliocentrism is the theory that the Sun is at the center of the Universe and/or the Solar System.

• Geocentric model (in Greek: geo = Earth and centron = center) of the Universe is a disproven model which places the Earth at the center of the universe.

Galileo Galileii

• Supported the ideas of Copernicus

•Worked for the Medici family

•Suffered house arrest the last 20 years of his life because of his support of the heliocentric universe theory

•Galileo’s work showed that forces were not necessary to keep an object moving.

Galileo on Motion

• A Force = ANY push or pull

• Friction = The force that acts between materials that touch as they move past each other.

• Caused by surface irregularities that touch

• Galileo argued that when friction is present, a force is required to keep an object moving !

Testing Galileo’s Theories

• If the slope is downward, speed increases

• The ball is rolling partly in the direction of Earth’s gravity

Testing Galileo’s Theories

• If the slope is upward, speed decreases

• The ball is rolling in a direction opposed to Earth’s gravity

Testing Galileo’s Theories

• If there is no slope, speed or velocity is constant

• The ball is not rolling with or against Earth’s gravity

! ?

A Second test of Galileo

• 2 inclined planes facing each other

• A ball released to roll down one plane will roll up the other to nearly the same height

A Second test of Galileo

• 2 inclined planes facing each other

• A ball released to roll down one plane will roll up the other to nearly the same height no matter what the angle of the second plane.

A Second test of Galileo

• 2 inclined planes facing each other, one reduced to zero or a flat plane

• A ball released to roll down one plane will roll forever with only friction to slow it.

Galileo’s Conclusions

• It is not the “nature” of the ball to come to rest as Aristotle had claimed.

• In the absence of friction, the moving ball would naturally keep moving.

• The tendency of a moving body to keep moving is called INERTIA

• (NEWTON’S 1st LAW!!!!!)

A New Vision of the Universe

• On Christmas Day 1642, (The same year Galileo died) Sir Isaac Newton (1642 – 1727) was born.

• By age 24 (1666), Newton had developed the Laws of Motion.

Newton’s First Law of Motion

• THE LAW OF INERTIA

• An object in motion tends to remain in motion at the same speed and in the same direction until acted upon by an external force. And an object at rest tends to remain at rest until acted upon by an external force.

Newton’s First Law of MotionThe Law of Inertia

• Examples:• Objects at rest – the ol’ tablecloth trick ….

• Objects in motion – a hockey puck or a table hockey disk

• Anything chucked out of the space station will move forever by virtue of its own inertia

INERTIA- amount of resistance to a change in velocity

NEWTON’S SECOND LAW OF MOTION

Mass – A Measure of Inertia

• Mass = The measure of how much matter

is present

The more mass an object has, the greater the inertia

The greater the inertia, the more force it takes to change an object’s state of inertia

Mass is NOT Volume

• Volume is a measure of the amount of space an object takes up.

• Volume is measured in cubic centimeters, cubic meters or liters

• Mass is measured in grams or kilograms

• Objects with large but equal volumes may have very different masses (and density!)

• EX : A bag of cloth and a bag of rocks

Which has more mass ?

Which is harder to set in motion ?

Which has the greater inertia ?

Wait !Mass is NOT Weight

• We often determine the amount of matter in an object (it’s mass) by measuring its gravitational attraction to the earth.

• Mass is more fundamental and doesn’t depend on gravity to be measurable.

• A bowling ball is just as difficult to shake in space as it is on the ground.

Force Causes Acceleration

• If a force is applied to an object, whether it is at rest or moving, the motion will change. IT ACCELERATES.

• If the force is removed, the object will continue moving at a constant velocity.

Newton’s Second Law of Motion

• The acceleration produced by a net force on an object is directly proportional to the magnitude of and in the same direction as the net force, and is inversely proportional to the mass of the object.

• acceleration = or a =net force

mass

Fm

FRICTION• Friction is a force that acts to oppose motion

between materials that are in contact with each other.

• Mainly due to irregularities between the two surfaces.

• When an object slides against another, it must rise over irregular bumps or scrape them off.

• Either way requires force.

• The force of friction depends on the types of materials that are in contact with each other and the amount of surface in contact.

• The shape of concrete dividers is such that the rubber of the tire against the concrete is more effective at slowing a car than steel barriers against the steel side of a car. And less damaging.

FRICTION

FREE FALL EXPLAINED• Galileo showed that falling objects accelerate equally…

• When air resistance is neglected… or

• When the air resistance is very small compared with the mass of the falling object. (but he didn’t know why)

• Galileo is said to have demonstrated this from the Leaning Tower of Pisa with a 10kg cannon ball and 1 kg stone.

NEWTON FIGURED IT OUT• Galileo knew that an object falls

because of the force due to gravity pulling on it.

• Sir Isaac Newton’s second law of motion showed the mass of a falling object must be considered as well.

• 10 times the force acting on 10 times the mass produces the same acceleration as a smaller force acting on a smaller mass.

OUCH !

F = mF = m..aa• Force = mass x acceleration

• Air resistance keeps things from falling at the same rate.

Styrofoam ball

golfball

Book and paper Flyingsquirrel

Newton’s Second Law

• F = m.a

NEWTON’S SECOND LAW OF MOTION

Be careful bending down !

• When you bend down, if you keep in contact with a wall you will be less likely to fall.

• This is because as you push on the wall for balance, the wall pushes back on you !

Forces and Interactions

• In simplest terms, a FORCE is a push or a pull.

• Sir Isaac Newton realized that a force is not a thing in itself but a part of a mutual action, an interaction between things.

• For example, The interaction that drives the nail is the same one that halts the hammer

Newton’s Third Law of Motion

• For every action there is an equal and opposite reaction.

• Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object.

Newton’s Third Law of Motion

• For every interaction, the forces always come in pairs (twos).

• The ACTION FORCE and

• The REACTION FORCE

• They are equal in strength and opposite in direction.

A simple recipe for treating action and reaction forces

• Identify the interaction:• ACTION : Object A exerts a force on object B

• REACTION : Object B exerts a force on object A

• For Example :• ACTION : Tire pushes road • REACTION : Road pushes tire

• ACTION : Rocket pushes gas• REACTION : Gas pushes rocket

Questions1. Does a stick of dynamite contain force?

No, force is not something an object has, like mass and volume. An object may posses the capability of exerting force on another object but it does not possess force.

2. A car accelerates along a road. Strictly speaking, what is the force that moves the car ?

It really is the road that pushes the car along ! The rotating tires push back on the road (action) and the road pushes forward on the tires (reaction) at the same time !

ACTION AND REACTION

Action and Reaction on Different Masses

• When a cannon is fired, there is an interaction between the cannon and the cannon ball.

• The forces the cannon ball and cannon exert on each other are equal and opposite.

• The cannonball moves fast while the cannon only Kicks a bit because of the difference in their masses.

OTHER ACTION – REACTION INTERACTIONS

• A Balloon

• A Rocket

• Or a jet

• Air foils : a bird’s wing,

• a helicopter blade

• an airplane wing

LIFT

WHAT’S IT MEAN ?

• If the action and reaction forces are internal to a system, they cancel each other and no acceleration occurs.

• For example : Blowing on a sail when you’re IN the boat doesn’t work very well.

• If either the action or the reaction force is external to the system, then acceleration occurs.

• However, the WIND blowing on the sail can work very well !

• FOR EVERY ACTION THERE IS AN EQUAL AND OPPOSITE REACTION

A fly gets smashed onto the windshield of a speeding bus.