Newton’s Laws of Motion 1 st – Law of Inertia 2 nd – F=ma 3 rd – Action-Reaction.

Newton’s Newton’s Laws of Laws of MotionMotion

11stst – – Law of InertiaLaw of Inertia

22ndnd – F=ma – F=ma

33rdrd – – Action-ReactionAction-Reaction

While most people While most people know what Newton's laws know what Newton's laws say, many people do not say, many people do not know what they mean (or know what they mean (or simply do not believe what simply do not believe what they mean). they mean).

Newton’s Laws of MotionNewton’s Laws of Motion

11stst Law Law:: An object at rest will stay An object at rest will stay at rest, and an object in motion will at rest, and an object in motion will stay in motion at constant speed and stay in motion at constant speed and direction (velocity), unless acted direction (velocity), unless acted upon by an unbalanced force.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 For every action there is an equal and opposite reaction.an equal and opposite reaction.

11stst Law of Motion Law of Motion (Law of Inertia) (Law of Inertia)

An object at rest will stay at An object at rest will stay at rest, and an object in motion rest, and an object in motion will stay in motion at will stay in motion at constant speed and direction constant speed and direction (velocity), unless acted upon (velocity), unless acted upon by an unbalanced force.by an unbalanced force.

Newton’s 1Newton’s 1stst Law Law Inertia is the Inertia is the

tendency of an tendency of an object to resist object to resist changes in its changes in its velocity: velocity: whether in whether in motion or motion or motionless.motionless.

These pumpkins will not move unless acted on by an unbalanced force.

Newton’s 1Newton’s 1stst Law Law Once airborne, Once airborne,

unless acted unless acted on by an on by an unbalanced unbalanced force (gravity force (gravity and air/fluid and air/fluid friction), it friction), it would never would never stop or change stop or change direction! direction!

Newton’s 1Newton’s 1stst 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.

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 motionless– seemingly without an outside force?without an outside force?It’s a force we sometimes cannot see–It’s a force we sometimes cannot see–

friction.friction.

Objects on earth, unlike Objects on earth, unlike the frictionless space the the frictionless space the moon travels through, are moon travels through, are under the influence of under the influence of 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?

Slide a book Slide a book across a table and across a table and watch it slide to a watch it slide to a resting position. resting position. The book comes to The book comes to a rest because of a rest because of the the presencepresence of a of a force—that force force—that force being the force of being the force of friction—which friction—which brings the book to brings the book to a rest position.a rest position.

In the absence of a force of In the absence of a force of friction, the book would continue friction, the book would continue in motion with the same speed in motion with the same speed and direction—forever! (Or at and direction—forever! (Or at least to the end of the table top.) least to the end of the table top.)

Newton’s 1Newton’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 Because of inertia, objects (including you) resist changes in their motion. you) resist changes in their motion. When the car going 80 km/hour is When the car going 80 km/hour is stopped by the brick wall, your body stopped by the brick wall, your body keeps moving at 80 m/hour until it keeps moving at 80 m/hour until it hits the steering wheel, windshield, or hits the steering wheel, windshield, or brick wall.brick wall.

Newton’s 2Newton’s 2ndnd Law Law


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


When mass is in kilograms and When mass is in kilograms and acceleration is in m/sacceleration is in m/s22, the unit of , the unit of force is in newtons (N).force is in newtons (N).

One newton is equal to the force One newton is equal to the force required to accelerate one kilogram required to accelerate one kilogram of mass at one meter/secondof mass at one meter/second22..

22ndnd Law (F = m Law (F = m • a) 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 • a Fill in given numbers and unitsFill in given numbers and units F = 1400 kg • 2 meters per second2

Solve for the unknownSolve for the unknown

2800 kg—meters /second2 or 2800 N

If mass remains constant, doubling the acceleration, doubles the force. If force remains constant, doubling the mass, halves the acceleration.

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/s98 N = 10 kg x 9.8 m/s22

F = maF = ma

9.8 N = 1 kg x 9.8 m/s9.8 N = 1 kg x 9.8 m/s22

Check Your UnderstandingCheck Your Understanding

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

2. A net force of 16 N causes a mass to accelerate 2. A net force of 16 N causes a mass to accelerate

at a rate of 5 m/sat a rate of 5 m/s22. Determine the mass.. Determine the mass.

3. How much force is needed to accelerate a 66 kg 3. How much force is needed to accelerate a 66 kg skier 1 m/secskier 1 m/sec22??

4. What is the force on a 1000 kg elevator that is 4. What is the force on a 1000 kg elevator that is falling freely at 9.8 m/secfalling freely at 9.8 m/sec22??

Check Your UnderstandingCheck Your Understanding1. What acceleration will result when a 12 N net 1. What acceleration will result when a 12 N net

force applied to a 3 kg object?force applied to a 3 kg object? 12 N = 3 kg x 4 m/s12 N = 3 kg x 4 m/s22

2. A net force of 16 N causes a mass to accelerate 2. A net force of 16 N causes a mass to accelerate at a rate of 5 m/sat a rate of 5 m/s22. Determine the mass.. Determine the mass.

16 N = 3.2 kg x 5 m/s16 N = 3.2 kg x 5 m/s22

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

66 kg-m/sec66 kg-m/sec22 or 66 N or 66 N

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

9800 kg-m/sec9800 kg-m/sec22 or 9800 N or 9800 N

Newton’s 3Newton’s 3rdrd Law Law

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


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


There are two There are two forces resulting forces resulting from this from this interaction—a force interaction—a force on the chair and a on the chair and a force on your body. force on your body. These two forces These two forces are called are called actionaction and and reactionreaction forces.forces.

Newton’s 3rd Law in Newton’s 3rd Law in NatureNature

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 by pushing the fish forwards, pushing the fish forwards, propelling the fish through propelling the fish through the water.the water.

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


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.

Consider the flying motion of birds. A bird Consider the flying motion of birds. A bird flies by use of its wings. The wings of a flies by use of its wings. The wings of a bird push air downwards. In turn, the air bird push air downwards. In turn, the air reacts by pushing the bird upwards. reacts by pushing the bird upwards.

The size of the force on the air equals the The size of the force on the air equals the size of the force on the bird; the direction size of the force on the bird; the direction of the force on the air (downwards) is of the force on the air (downwards) is opposite the direction of the force on the opposite the direction of the force on the bird (upwards).bird (upwards).

Action—reaction force pairs make it Action—reaction force pairs make it possible for birds to fly.possible for birds to fly.

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

The baseball forces The baseball forces the bat to the left the bat to the left ((an actionan action); the bat ); the bat forces the ball to forces the ball to the right (the right (the the reactionreaction). ).

Newton’s Newton’s 33rdrd Law Law Consider the motion of a Consider the motion of a

car on the way to car on the way to school. A car is school. A car is equipped with wheels equipped with wheels which spin. As the which spin. As the wheels spin, they grip wheels spin, they grip the road and push the road and push against it road (against it road (action action forceforce). The road pushes ). The road pushes back on the tires to back on the tires to propel the car forward propel the car forward ((reaction forcereaction force).).

Newton’s Newton’s 33rdrd Law LawThe reaction of a rocket The reaction of a rocket is an application of the is an application of the third law of motion. third law of motion. Various fuels are burned Various fuels are burned in the engine, producing in the engine, producing hot gases. hot gases.

The hot gases push The hot gases push against the inside tube of against the inside tube of the rocket and escape out the rocket and escape out the bottom of the tube. the bottom of the tube. As the gases move As the gases move downward, the rocket downward, the rocket moves in the opposite moves in the opposite direction.direction.

Newton’s Laws of Motion 1 st – Law of Inertia 2 nd – F=ma 3 rd – Action-Reaction.

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