Newton’s Laws

Three Laws of MotionThree Laws of Motion

Aristotle’s Motion

Natural Motion is up or downNatural Motion is up or down Down for falling objectsDown for falling objects Up for smokeUp for smoke Circular for heavenly bodies since without endCircular for heavenly bodies since without end

Violent Motion Violent Motion Due to imposed forces such as wind pushing a Due to imposed forces such as wind pushing a

ship or someone pulling a cartship or someone pulling a cart Natural state of motion is restNatural state of motion is rest

A force is needed to keep something movingA force is needed to keep something moving

Aristotle’s Basic Error

Friction not understood as a forceFriction not understood as a force

Galileo’s Motion

Force is a push or a pullForce is a push or a pull Friction is a force that Friction is a force that

occurs when objects move occurs when objects move past each otherpast each other

Friction due to tiny Friction due to tiny irregularitiesirregularities

OnlyOnly when friction is when friction is present is a force required present is a force required to keep something movingto keep something moving

Galileo’s Inclined Planes

Ball rolling downhill Ball rolling downhill speeds upspeeds up

Ball rolling uphill Ball rolling uphill slows downslows down

He asked about ball on He asked about ball on smooth level surfacesmooth level surface

Concluded it would Concluded it would roll forever in absence roll forever in absence of frictionof friction

Inertia

Resistance to change in state of motionResistance to change in state of motion Galileo concluded all objects have inertiaGalileo concluded all objects have inertia Contradicted Aristotle’s theory of motionContradicted Aristotle’s theory of motion No force required to keep Earth in motion No force required to keep Earth in motion

around sun because no frictionaround sun because no friction

Newton

Born 1665Born 1665 Built on Galileo’s Built on Galileo’s

ideasideas Proposed three laws of Proposed three laws of

motion at age of 23motion at age of 23

Newton’s First Law

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

Also called Law of Inertia: things move according Also called Law of Inertia: things move according to their own inertiato their own inertia

Things keep on doing what they are doingThings keep on doing what they are doing Examples: Hockey puck on ice, rolling ball, ball Examples: Hockey puck on ice, rolling ball, ball

in spacein space

Ourtesy www.lakeheadu.ca/~alumni/ hockey.gif

Mass

Amount of inertia depends on amount of Amount of inertia depends on amount of mass…or amount of material (number and mass…or amount of material (number and kind of atoms)kind of atoms)

Measured in kilogramsMeasured in kilograms Question: Which has more mass, a Question: Which has more mass, a

kilogram of lead or a kilogram of feathers?kilogram of lead or a kilogram of feathers? Mass vs. Volume: volume is how much Mass vs. Volume: volume is how much

space something occupiesspace something occupies

Experiencing Inertia

Inertia is resistance to shakingInertia is resistance to shaking Which is easier to shake, a pen or a person?Which is easier to shake, a pen or a person? Why is it so hard to stop a heavy boat?Why is it so hard to stop a heavy boat?

Mass vs. Weight

Mass is intrinsic property of any objectMass is intrinsic property of any object Weight measures gravitational force on an Weight measures gravitational force on an

object, usually due to a planetobject, usually due to a planet Weight depends on location of objectWeight depends on location of object Question 1: How does mass of a rock Question 1: How does mass of a rock

compare when on Earth and on moon?compare when on Earth and on moon? Question 2: How does its weight compare?Question 2: How does its weight compare?

Review Mass vs. Weight

What is mass?What is mass? Answer: quantity of Answer: quantity of

matter in something or matter in something or a measure of its inertiaa measure of its inertia

What is weight?What is weight? Answer: Force on a Answer: Force on a

body due to gravitybody due to gravity

Weight of 1 Kilogram

9.8 Newtons9.8 Newtons About 2.2 poundsAbout 2.2 pounds Compare the weight of 1 kg nails with 1 kg Compare the weight of 1 kg nails with 1 kg

styrofoamstyrofoam Answer: SameAnswer: Same

Weight Examples

What does a 70 kg person weigh?What does a 70 kg person weigh?

Weight = mass x g(acceleration due to Weight = mass x g(acceleration due to gravity)gravity)

W = mg = 70 kg x 9.80 N/mW = mg = 70 kg x 9.80 N/m2 =2 = 686 N 686 N An object weighs 9800 n on Earth. What is An object weighs 9800 n on Earth. What is

its mass?its mass? m = W/g = 9800 / 9.8 m/sm = W/g = 9800 / 9.8 m/s2 2 == 1000 kg1000 kg

Inertia in a Car Discuss three examples of inertia in a carDiscuss three examples of inertia in a car

•Car hitting a wall

•Car hit from behind by a truck

•Car going around a corner

Newton’s Second Law

Law of AccelerationLaw of Acceleration The acceleration produced by a net force on The acceleration produced by a net force on

an object is directly proportional to the an object is directly proportional to the magnitude of the net force, and is inversely magnitude of the net force, and is inversely proportional to the mass of the body.proportional to the mass of the body.

Acceleration = net force Acceleration = net force ÷mass÷mass F =maF =ma Acceleration is Acceleration is in direction of net forcein direction of net force

Net Force

Net Force means sum of all forces actingNet Force means sum of all forces acting Sum is Vector sumSum is Vector sum

F1

F2

Resultant force

Understanding the Second Law

The cause of acceleration is…The cause of acceleration is… _________ resists acceleration_________ resists acceleration The greater the force, the ________ the The greater the force, the ________ the

____________________________ The greater the mass, the _________ the The greater the mass, the _________ the

acceleration.acceleration.

Force

Mass or inertia

acceleration

less

greater

Units

F = maF = ma Unit of force is the Newton (N)Unit of force is the Newton (N) 1 N = 1 kg m/s1 N = 1 kg m/s22

F = ma is Three Equations

F F andand a a are vectorsare vectors So So FF = m = maa equation is really three equation is really three

FFxx = ma = max x FFyy = ma = may y FFzz = ma = mazz

Examples

What force is required to accelerate a 1000 What force is required to accelerate a 1000 kg car at 2.0 m/skg car at 2.0 m/s2 2 ??

Answer: F = ma = 1000 kg x 2.0 m/sAnswer: F = ma = 1000 kg x 2.0 m/s2 2 = = 2000 N.2000 N.

What is the acceleration of a 145 g baseball What is the acceleration of a 145 g baseball thrown with a force of 20.0 N?thrown with a force of 20.0 N?

a = F/m = 20.0 N/0.145kg = 138 m/sa = F/m = 20.0 N/0.145kg = 138 m/s22

F = ma Example; m unknown

An astronaut puts a 500.0 N force on an An astronaut puts a 500.0 N force on an object of unknown mass producing an object of unknown mass producing an accelerations of 0.462 m/saccelerations of 0.462 m/s2 2 . What was the . What was the mass?mass?

M = F/a = 500.0N/0.462 m/sM = F/a = 500.0N/0.462 m/s2 2 = 1082 Kg = = 1082 Kg = 1.08 x 101.08 x 103 3 KgKg

Newton’s Third Law

Forces always come in pairsForces always come in pairs Two forces on different objectsTwo forces on different objects Whenever one object exerts a force on a Whenever one object exerts a force on a

second object, the second exerts an equal second object, the second exerts an equal and opposite force on the firstand opposite force on the first

Example: hammer hits nailExample: hammer hits nail

Example: pushing on wall

What are the forces when you push on a What are the forces when you push on a wall?wall?

You exert force on wallYou exert force on wall You accelerate in the opposite directionYou accelerate in the opposite direction Wall must have exerted a force on you in Wall must have exerted a force on you in

the direction you accelerated (by 2the direction you accelerated (by 2ndnd Law) Law)

Example: person walking

Foot exerts force Foot exerts force backward on groundbackward on ground

Ground exerts force Ground exerts force forward on footforward on foot

Example: Throwing ball

Pitcher exerts force on Pitcher exerts force on ballball

Ball exerts equal and Ball exerts equal and opposite force on opposite force on pitcherpitcher

Why doesn’t pitcher Why doesn’t pitcher move?move?

Example: Rocket

Rocket engine exerts Rocket engine exerts rearward force on gas rearward force on gas moleculesmolecules

Molecules exert Molecules exert forward force on forward force on rocket.rocket.

Horse and Cart

Horse exerts force on Horse exerts force on cartcart

Cart exerts equal and Cart exerts equal and opposite force on opposite force on horsehorse

Net force = zeroNet force = zero Cart can’t moveCart can’t move Huh?Huh?

Book on Table

The mass of the book is one kg. What is The mass of the book is one kg. What is the force (magnitude and direction) the force (magnitude and direction) onon the the book?book?

9.8 N upward9.8 N upward

More Examples

Can you think of some more examples of Can you think of some more examples of Newton’s Third Law in Action?Newton’s Third Law in Action?

Review: Newton’s Laws of Motion

Newton’s First Law:Newton’s First Law: Every object continues in its state of rest, or of motion in a Every object continues in its state of rest, or of motion in a

straight line at constant speed, unless compelled to change straight line at constant speed, unless compelled to change that state by forces exerted on it.that state by forces exerted on it.

Newton’s Second Law:Newton’s Second Law: The acceleration produced by a net force on an object is The acceleration produced by a net force on an object is

directly proportional to the magnitude of the net force, and directly proportional to the magnitude of the net force, and is inversely proportional to the mass of the body.is inversely proportional to the mass of the body.

Newton’s Third Law:Newton’s Third Law: Whenever one object exerts a force on a second object, the Whenever one object exerts a force on a second object, the

second exerts an equal & opposite force on the firstsecond exerts an equal & opposite force on the first