Essential Question

• How can I explain the causes of motion on an

object?

• Learning Objectives: Laws of Motion, Friction, Force

BackgroundSir Isaac Newton (1643-1727) an English

scientist and mathematician famous for his discovery of the law of gravity also

discovered the three laws of motion. He published them in his book Philosophiae

Naturalis Principia Mathematica (mathematic principles of natural

philosophy) in 1687. Today these laws are known as Newton’s Laws of Motion and describe the motion of all objects on the

scale we experience in our everyday lives.

Newton’s Laws of MotionNewton’s Laws of Motion

• 11stst Law Law – An object at rest will stay at – An object at rest will stay at rest, and an object in motion will stay rest, and an object in motion will stay in motion at constant velocity, unless in motion at constant velocity, unless acted upon by an unbalanced force.acted upon by an unbalanced force.

• 22ndnd Law Law – Force equals mass times – Force equals mass times acceleration.acceleration.

• 33rdrd Law Law – For every action there is an – For every action there is an equal and opposite reaction.equal and opposite reaction.

First Law: An object at rest stays at rest or an object in motion, stays in motion (in the same direction/at the same speed) unless acted upon by an unbalanced force

Also called the law of inertia

InertiaInertia

A property of matterThe tendency of an object to resist any

change in its motionThe greater the mass the greater the inertiaThe greater the speed the greater the inertia

Examples of Newton’s 1st Law

a) car suddenly stops and you strain against the seat belt b) when riding a horse, the horse suddenly stops and you fly over its head c) the magician pulls the tablecloth out from under a table full of dishes d) the difficulty of pushing a dead car e) lawn bowling on a cut and rolled lawn verses an uncut lawn appearf) car turns left and you to slide to the right

Examples of Newton’s 1st Law

11stst Law Law

• Once airborne, Once airborne, unless acted on unless acted on by an by an unbalanced unbalanced force (gravity force (gravity and air – fluid and air – fluid friction), it would friction), it would never stop! never stop!

11stst Law Law

• Unless acted Unless acted upon by an upon by an unbalanced unbalanced force, this golf force, this golf ball would sit ball would sit on the tee on the tee forever. forever.

Newton’s First Law

• Newton’s First Law of Motion– An object at rest will remain at rest and

an object in motion will continue moving at a constant velocity unless acted upon by a net force.

Newton’s First Law• Newton’s First Law of Motion

– “Law of Inertia”

• Inertia– tendency of an object to resist any change in its

motion– increases as mass increases

Some Examples from Real Life

Two teams are playing tug of war. They are both exerting equal force on the rope in opposite directions. This balanced force results in no change of motion.

A soccer ball is sitting at rest. It takes an unbalanced force of a kick to change its motion.

More Examples from Real LifeA powerful locomotive begins to pull a long line of boxcars that were sitting at rest. Since the boxcars are so massive, they have a great deal of inertia and it takes a large force to change their motion. Once they are moving, it takes a large force to stop them.

On your way to school, a bug flies into your windshield. Since the bug is so small, it has very little inertia and exerts a very small force on your car (so small that you don’t even feel it).

If objects in motion tend to stay in motion, why don’t moving objects keep moving

forever?

Things don’t keep moving forever because there’s almost always an unbalanced force

acting upon it.

A book sliding across a table slows down and stops because of the force of friction.

If you throw a ball upwards it will eventually slow down and fall because of the force of gravity.

Why then, do we observe Why then, do we observe every day objects in motion every day objects in motion slowing down and becoming slowing down and becoming motionless seemingly without motionless seemingly without an outside force?an outside force?

It’s a force we sometimes cannot see – It’s a force we sometimes cannot see – friction.friction.

• There are four main types of friction:There are four main types of friction:– Sliding friction: Sliding friction: ice skating– Rolling friction: Rolling friction: bowling– Fluid friction (air or liquid): Fluid friction (air or liquid): air or water resistance– Static friction: Static friction: initial friction when moving an object

What is this unbalanced force that acts on an What is this unbalanced force that acts on an object in motion?object in motion?

Newton’s First Law

An object at rest tends to stay at rest and an object in motion tends to stay

in motion unless acted upon by an unbalanced force.

What does this mean? Review…

Basically, an object will “keep doing what it was doing” unless acted on by an unbalanced force.

If the object was sitting still, it will remain stationary. If it was moving at a constant velocity, it will keep moving.

It takes force to change the motion of an object.

Newtons’s 1Newtons’s 1stst Law and You Law and You

Don’t let this be you. Wear seat belts.Don’t let this be you. Wear seat belts.

Because of inertia, objects (including you) Because of inertia, objects (including you) resist changes in their motion. When the resist changes in their motion. When the car going 80 km/hour is stopped by the car going 80 km/hour is stopped by the brick wall, your body keeps moving at 80 brick wall, your body keeps moving at 80 m/hour.m/hour.

22ndnd Law Law

The net force of an object The net force of an object is equal to the product of its is equal to the product of its mass and acceleration, or mass and acceleration, or F=ma.F=ma.

Second law: The greater the force applied to an object, the more the object will accelerate. It takes more force to accelerate an object with a lot of mass than to accelerate something with very little mass.

The player in black had more acceleration thus he hit with a

greater amount of force

22ndnd Law Law

What is Force? It is any push or pull on an object!!

Second law:The greater the force, the greater the accelerationThe greater the mass, the greater the force needed

for the same accelerationCalculated by: F = ma (F = force, m = mass, a = acceleration)

Examples of Newton’s 2nd Law

a) hitting a baseball, the harder the hit, the faster the ball goes b) accelerating or decelerating a car c) The positioning of football players - massive players on the line with lighter (faster to accelerate) players in the backfield d) a loaded versus an unloaded truck

Examples of Newton’s 2nd Law

Newton’s Second Law

• Newton’s Second Law of Motion– The acceleration of an object is directly proportional

to the net force acting on it and inversely proportional to its mass.

F = ma

Newton’s Second Law

Force equals mass times acceleration.

F = ma

Acceleration: a measurement of how quickly an object is changing speed.

22ndnd Law Law

• When mass is in kilograms and When mass is in kilograms and acceleration is in m/s/s, the unit of force is acceleration is in m/s/s, the unit of force is in newtons (N).in newtons (N).

• One newton is equal to the force required One newton is equal to the force required to accelerate one kilogram of mass at one to accelerate one kilogram of mass at one meter/second/second.meter/second/second.

22ndnd Law (F = m x a) Law (F = m x a)

• How much force is needed to accelerate a 1400 kilogram car 2 meters per second/per second?

• Write the formulaWrite the formula• F = m x a• Fill in given numbers and unitsFill in given numbers and units• F = 1400 kg x 2 meters per second/second• Solve for the unknownSolve for the unknown

• 2800 kg-meters/second/second or 2800 N

Newton’s 2nd Law proves that different masses accelerate to the earth at the same rate, but with different forces.

• We know that objects with different masses accelerate to the ground at the same rate.

• However, because of the 2nd Law we know that they don’t hit the ground with the same force.

F = maF = ma

98 N = 10 kg x 9.8 m/s/s98 N = 10 kg x 9.8 m/s/s

F = maF = ma

9.8 N = 1 kg x 9.8 9.8 N = 1 kg x 9.8 m/s/sm/s/s

ConceptTest• Is the following statement true or false?

– An astronaut has less mass on the moon since the moon exerts a weaker gravitational force.

– False! Mass does not depend on gravity, weight does. The astronaut has less weight on the moon.

Check Your UnderstandingCheck Your Understanding

• 1. What acceleration will result when a 12 N net force applied to a 3 1. What acceleration will result when a 12 N net force applied to a 3 kg object? A 6 kg object?kg object? A 6 kg object?

  • 2. A net force of 16 N causes a mass to accelerate at a rate of 5 2. A net force of 16 N causes a mass to accelerate at a rate of 5

m/sm/s22. Determine the mass.. Determine the mass.

• 3. How much force is needed to accelerate a 66 kg skier 1 3. How much force is needed to accelerate a 66 kg skier 1 m/sec/sec?m/sec/sec?

• 4. What is the force on a 1000 kg elevator that is falling freely at 9.8 4. What is the force on a 1000 kg elevator that is falling freely at 9.8 m/sec/sec?m/sec/sec?

third law: For every action force, there is an equal and opposite reaction force. (Forces are always paired)

33rdrd Law Law

According to Newton, According to Newton, whenever objects A whenever objects A and B interact with and B interact with each other, they exert each other, they exert forces upon each forces upon each other. When you sit in other. When you sit in your chair, your body your chair, your body exerts a downward exerts a downward force on the chair and force on the chair and the chair exerts an the chair exerts an upward force on your upward force on your body. body.

Newton’s 3rd Law in NatureNewton’s 3rd Law in Nature

• Consider the propulsion of Consider the propulsion of a fish through the water. A a fish through the water. A fish uses its fins to push fish uses its fins to push water backwards. In turn, water backwards. In turn, the water the water reactsreacts by pushing by pushing the fish forwards, propelling the fish forwards, propelling the fish through the water.the fish through the water.

• The size of the force on The size of the force on the water equals the size of the water equals the size of the force on the fish; the the force on the fish; the direction of the force on the direction of the force on the water (backwards) is water (backwards) is opposite the direction of the opposite the direction of the force on the fish (forwards).force on the fish (forwards).

33rdrd Law Law

Flying gracefully Flying gracefully through the air, through the air, birds depend on birds depend on Newton’s third Newton’s third law of motion. As law of motion. As the birds push the birds push down on the air down on the air with their wings, with their wings, the air pushes the air pushes their wings up their wings up and gives them and gives them lift.lift.

Examples of Newton’s 3rd Law

a)rockets leaving earthb)guns being firedc) two cars hit head on d) astronauts in space e) pool or billiards f) jumping out of a boat onto the dock g) sprinklers rotating

Examples of Newton’s 3rd LawNewton’s third law: "For

every action, there is an equal and opposite reaction." When you fire a gun you feel the recoil. Some of the funniest things in cartoons follow physics that have been exaggerated or just plain ignored. Wyle Coyote hangs suspended in space over that canyon for a lot longer than an object would in reality, but it is the anticipation of the drop and Wyle's facial recognition of the upcoming pain that is so classically cartooney. So some laws are stretched for comical effect.

Other examples of Newton’s Other examples of Newton’s Third LawThird Law

• The baseball forces The baseball forces the bat to the left (an the bat to the left (an action); the bat forces action); the bat forces the ball to the right the ball to the right (the reaction). (the reaction).

33rdrd Law Law

• Consider the motion Consider the motion of a car on the way to of a car on the way to school. A car is school. A car is equipped with wheels equipped with wheels which spin which spin backwards. As the backwards. As the wheels spin wheels spin backwards, they grip backwards, they grip the road and push the the road and push the road backwards.road backwards.

Examples of Newton’s 3rd Law

Newton’s Third Law

• Newton’s Third Law of Motion– When one object exerts a force on a

second object, the second object exerts an equal but opposite force on the first.

Newton’s Third Law• Problem:

– How can a horse pull a cart if the cart is pulling back on the horse with an equal but opposite force?

NO!!!

– Aren’t these “balanced forces” resulting in no acceleration?

Newton’s Third Law

– forces are equal and opposite but act on different objects

– they are not “balanced forces”– the movement of the horse depends on the

forces acting on the horse

• Explanation:

Newton’s Third Law

• Action-Reaction Pairs

• The hammer exerts a force on the nail to the right.

• The nail exerts an equal but opposite force on the hammer to the left.

Newton’s Third Law

• Action-Reaction Pairs• The rocket exerts a

downward force on the exhaust gases.

• The gases exert an equal but opposite upward force on the rocket.

FG

FR

Newton’s Third Law

For every action there is an equal and opposite reaction.

What does this mean?

For every force acting on an object, there is an equal force acting in the opposite direction. Right now, gravity is pulling you down in your seat, but Newton’s Third Law says your seat is pushing up against you with equal force. This is why you are not moving. There is a balanced force acting on you– gravity pulling down, your seat pushing up.

Think about it . . .

What happens if you are standing on a skateboard or a slippery floor and push against a wall? You slide in the opposite direction (away from the wall), because you pushed on the wall but the wall pushed back on you with equal and opposite force.

Why does it hurt so much when you stub your toe? When your toe exerts a force on a rock, the rock exerts an equal force back on

your toe. The harder you hit your toe against it, the more force the rock exerts back on your toe (and the more your toe

hurts).

ReviewNewton’s First Law:

Objects in motion tend to stay in motion and objects at rest tend to stay at rest unless acted upon by an unbalanced force.

Newton’s Second Law:

Force equals mass times acceleration (F = ma).

Newton’s Third Law:

For every action there is an equal and opposite reaction.

Newton’s Laws of Motion

• An object in motion tends to stay in motion and an object at rest tends to stay at rest unless acted upon by an unbalanced force.

• Force equals mass times acceleration

(F = ma).• For every action there is an equal

and opposite reaction.

Which of Newton’s Laws of Motion apply to these situations?

• An object rests in your hand • A ball is tossed into the air• A car windshield hits a bug• A person sits on a table• A person jumps up from the floor• A baseball bat hits a baseball• A truck and car hit head on• Any others?

--http://smartweed.olemiss.edu/nmgk8/curriculum/middleschool/eighth/discovering_newtons_laws/Discovering%20Newtons%20Laws.ppt

http://www3.science.tamu.edu/CMSE/powerpoint/Newton'sLaws.pptv