Newton’s Newton’s Laws of MotionLaws of Motion

II Law of Inertia Law of Inertia

IIII F=MA F=MA

IIIIII 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 – – 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.

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

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

11stst Law LawInertia 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

The moon will keep The moon will keep revolving around revolving around the earth forever, the earth forever, unless acted on by unless acted on by an unbalanced an unbalanced force.force.

• Moon in orbit around earth

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 – fluid and air – 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 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 skatingice skating– Rolling friction: Rolling friction: bowlingbowling– Fluid friction (air or liquid): Fluid friction (air or liquid): air or water resistanceair or water resistance– Static friction: Static friction: initial friction when moving an initial friction when moving an

objectobject

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

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

22ndnd 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.

22ndnd Law Law

When mass is in kilograms and When mass is in kilograms and acceleration is in m/s/s, the unit of acceleration is in m/s/s, 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/second/second.of mass at one meter/second/second.

22ndnd Law (F=MxA) Law (F=MxA)How much force is needed to accelerate a How much force is needed to accelerate a 1400 Kilogram car 2 meters per second/per 1400 Kilogram car 2 meters per second/per second?second?Write the formulaWrite the formulaF = M x AF = M x AFill in given numbers and unitsFill in given numbers and unitsF = 1400K x 2 meters per second/secondF = 1400K x 2 meters per second/secondSolve for the unknownSolve for the unknown

2800 K-meters/second/second or 2800 K-meters/second/second or 2800 2800 NN

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 rate 2. A net force of 16 N causes a mass to accelerate at a rate of 5 m/s2. Determine the mass.of 5 m/s2. 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 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 1. What acceleration will result when a 12-N net force applied to a 3-kg object? 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. 2. A net force of 16 N causes a mass to accelerate at a rate of 5 m/s2. Determine the mass.Determine the 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 4. What is the force on a 1000-kg elevator that is falling freely at 9.8 m/sec/sec.?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 For every action, there is an equal and opposite an equal and opposite reaction.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 Law

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 Newton’s 3rd Law in NatureLaw in NatureConsider 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; the of the force on the fish; the direction of the force on direction of the force on the water (backwards) is the water (backwards) is opposite the direction of opposite the direction of the force on the fish the force on the fish (forwards).(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 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 action); the bat (an action); the bat forces the ball to forces the ball to the right (the the right (the reaction). reaction).

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