Newton’s Newton’s Laws of Laws of MotionMotionI.I. Law of Inertia Law of Inertia

II.II. F=ma F=maIII.III. Action-Reaction Action-Reaction

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

Newton’s Laws of MotionNewton’s Laws of Motion 11stst Law Law – An object at rest will – An object at rest will stay at rest, and an object in stay at rest, and an object in motion will stay in motion at motion will stay in motion at constant velocity, unless acted constant 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 For every action there is an equal and opposite reaction.is 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 constant will stay in motion at constant velocity, unless acted upon by velocity, unless acted upon by an unbalanced force.an unbalanced force.

11stst 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: whether velocity: whether in motion or in motion or motionless.motionless. These pumpkins will not move

unless acted on by an unbalanced force.

11stst Law Law Once airborne, Once airborne, unless acted unless acted on by an on by an unbalanced unbalanced force (gravity force (gravity and air – and air – fluid fluid friction), it friction), it would never would never stop! stop!

11stst Law Law Unless acted Unless acted upon by an upon by an unbalanced unbalanced force, this force, this golf ball golf ball would sit on would sit on the tee the tee forever. forever.

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

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

Objects on earth, unlike Objects on earth, unlike the frictionless space the moon the frictionless space the moon travels through, are under the travels through, are under the influence of friction.influence of 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 watch it slide to a rest position. a rest 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 friction, the book would continue in motion with the continue in motion with the same speed and direction - same speed and direction - forever! (Or at least to the forever! (Or at least to the end of the table top.) end of the table top.)

Newtons’s 1Newtons’s 1stst Law and Law and YouYou

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. When you) 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 the brick wall, your body keeps moving at 80 m/hour.80 m/hour.

22ndnd Law Law

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.

22ndnd Law Law

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

One newton is equal to the force One newton is equal to the force required to accelerate one required to accelerate one kilogram of mass at 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

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 = ma98 N = 10 kg x 9.8 98 N = 10 kg x 9.8

m/s/sm/s/s

F = maF = ma9.8 N = 1 kg x 9.8 N = 1 kg x

9.8 m/s/s9.8 m/s/s

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 applied to a 3 kg object? A 6 kg object?force applied to a 3 kg object? A 6 kg 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/sec/sec?skier 1 m/sec/sec?

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?

Check Your UnderstandingCheck Your Understanding 1. What acceleration will result when a 12 N net force applied to 1. What acceleration will result when a 12 N net force applied to

a 3 kg object? a 3 kg object? 12 N = 3 kg x 4 m/s/s12 N = 3 kg x 4 m/s/s

   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. 16 N = 3.2 kg 16 N = 3.2 kg

x 5 m/s/sx 5 m/s/s  

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?

66 kg-m/sec/sec or 66 N66 kg-m/sec/sec or 66 N

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

 9800 kg-m/sec/sec or 9800 N9800 kg-m/sec/sec or 9800 N

33rdrd Law LawFor every action, For every action, there is an equal and there is an equal and opposite reaction.opposite reaction.

33rdrd Law LawAccording to According to Newton, whenever Newton, whenever objects A and B objects A and B interact with each interact with each other, they exert other, they exert forces upon each forces upon each other. When you sit other. When you sit in your chair, your in your chair, your body exerts a 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 upward force on your body. your body.

33rdrd Law LawThere are two There are two forces resulting forces resulting from this from this interaction - a interaction - a force on the force on the chair and a force chair and a force on your body. on your body. These two forces These two forces are called are called actionaction and and reactionreaction forces.forces.

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 fish through the water. A fish uses its fins to A fish uses its fins to push water backwards. In push water backwards. In turn, the water turn, the water reactsreacts by by pushing the fish forwards, pushing the fish forwards, propelling the fish propelling the fish through the water.through 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 is opposite the direction of the force on the fish of the force on the fish (forwards).(forwards).

33rdrd Law LawFlying 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 Consider the flying motion of birds. A bird flies by use of its wings. The bird flies by use of its wings. The wings of a bird push air downwards. In wings of a bird push air downwards. In turn, the air reacts by pushing the turn, the air reacts by pushing the bird upwards. bird upwards.

The size of the force on the air equals The size of the force on the air equals the size of the force on the bird; the the size of the force on the bird; the direction of the force on the air direction of the force on the air (downwards) is opposite the direction (downwards) is opposite the direction of the force on the bird (upwards).of the force on the 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 The baseball forces the bat forces the bat to the left (an to the left (an action); the bat action); the bat forces the ball forces the ball to the right to the right (the reaction). (the reaction).

33rdrd Law Law Consider the Consider the motion of a car on motion of a car on the way to school. the way to school. A car is equipped A car is equipped with wheels which with wheels which spin backwards. As spin backwards. As the wheels spin the wheels spin backwards, they backwards, they grip the road and grip the road and push the road push the road backwards.backwards.

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

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