Newton’s Newton’s Laws of Laws of MotionMotion

I.I. Inertia Inertia

II.II. Acceleration Acceleration

III.III. Action-Reaction Action-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 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 – Acc – Acceleration is directly eleration is directly proportional to Force but inversely proportional to Force but inversely proportional to mass.proportional to mass.

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

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

An object at rest will stay An object at rest will stay at rest, and an object in at rest, and an object in motion will stay in motion motion will stay in motion at constant velocity, at constant velocity, unless acted upon by an unless acted upon by an unbalanced force.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: velocity: whether in whether in motion or 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 on unless acted on by an by an unbalanced force unbalanced force (gravity and air (gravity and air – fluid friction), – fluid friction), it would never it would never stop! 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 on ball would sit on the tee forever. the tee 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 without an motionless seemingly without an outside force?outside force?

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

Objects on earth, unlike the Objects on earth, unlike the frictionless space the moon 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 What is this unbalanced force that acts on an object in motion?acts on an 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 rest position. The rest position. The book comes to a book comes to a rest because of the rest because of the presencepresence of a force of a force - that force being - that force being the force of friction the force of friction - which brings the - which brings the book to a rest book to a rest position.position.

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

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

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

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

The truck is in motion. What is the force that causes it to stop?

The push of the stopped car.

The car is at rest. What is the force that causes it to move?

The push of the truck.

Slide from www.science-class.net

Newton's Second LawNewton's Second Law

If you apply If you apply moremore force to force to an object, it an object, it accelerates at accelerates at a higher rate.a higher rate.

5.2 Newton's Second Law5.2 Newton's Second Law

If the same force is If the same force is applied to an object applied to an object with greater mass, with greater mass, the object the object accelerates at a accelerates at a slower rate because slower rate because mass adds inertia.mass adds inertia.

22ndnd Law Law

22ndnd Law Law

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

22ndnd Law Law

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

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

Three forms of the second law:Three forms of the second law:

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 = 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

Check Your UnderstandingCheck Your Understanding

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

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

rate of 5 m/srate of 5 m/s22. Determine the mass.. Determine the mass.

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

4. What is the force on a 1000 kg elevator that is falling 4. What is the force on a 1000 kg elevator that is falling freely at 9.8 m/sec/sec?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 a 3 kg object? 1. What acceleration will result when a 12 N net force applied to 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 m/s2. A net force of 16 N causes a mass to accelerate at a rate of 5 m/s22. Determine the . Determine the

mass.mass. 16 N = 3.2 kg x 5 m/s/s16 N = 3.2 kg x 5 m/s/s

   3. How much force is needed to accelerate a 66 kg skier 1 m/sec/sec?3. How much force is needed to accelerate a 66 kg skier 1 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 at 9.8 m/sec/sec?4. What is the force on a 1000 kg elevator that is falling freely at 9.8 m/sec/sec?

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

33rdrd Law Law

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

33rdrd Law Law

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

33rdrd Law Law

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

5.3 Forces occur in pairs5.3 Forces occur in pairs

The two forces in a pair are The two forces in a pair are called called actionaction and and reactionreaction. .

Anytime you have one, you Anytime you have one, you also have the other. also have the other.

If you know the strength of one If you know the strength of one you also know the strength of you also know the strength of the other since both forces are the other since both forces are always equal.always equal.

Newton’s Third Law of MotionNewton’s Third Law of Motion

If one object exerts a force on If one object exerts a force on another object, then the another object, then the second object exerts a force second object exerts a force of equal strength in the of equal strength in the opposite direction on the opposite direction on the first objectfirst object

““For every action there is an For every action there is an equal but opposite reaction”equal but opposite reaction”

Action – Reaction PairsAction – Reaction Pairs

Kayak paddle pushes the water Kayak paddle pushes the water back, the water pushes the kayak back, the water pushes the kayak forwardforward

A dog pushes down on the A dog pushes down on the ground, the ground pushes up on ground, the ground pushes up on the dog causing it to leap into the the dog causing it to leap into the airair

Other Examples…….Other Examples…….

Newton’s 3rd Law in NatureNewton’s 3rd Law in Nature Consider the propulsion of a Consider the propulsion of a

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

The size of the force on the The size of the force on the water equals the size of the water equals the size of the force on the fish; 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.

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

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

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

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

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

33rdrd Law Law

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

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.

Do Action – Reaction Forces Do Action – Reaction Forces Cancel?Cancel?

If 2 equal act in opposite directions they are If 2 equal act in opposite directions they are balanced and cancel each other out. There is no balanced and cancel each other out. There is no movementmovement

Action – Reaction forces do not cancel out Action – Reaction forces do not cancel out because they are acting on 2 different objectsbecause they are acting on 2 different objects

Thank You!Thank You!