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Newton's laws of motion imply the relationship between an object's motion and the forces acting on it.

1.
Laws of

Motion

I. Law of Inertia

II. F=ma

III. Action-Reaction

Motion

I. Law of Inertia

II. F=ma

III. Action-Reaction

2.
While most people know

what Newton's laws say,

many people do not know

what they mean (or simply do

not believe what they mean).

what Newton's laws say,

many people do not know

what they mean (or simply do

not believe what they mean).

3.
Newton’s Laws of Motion

1st Law – An object at rest will stay at rest,

and an object in motion will stay in

motion at constant velocity, unless acted

upon by an unbalanced force.

2nd Law – Force equals mass times

acceleration.

3rd Law – For every action there is an

equal and opposite reaction.

1st Law – An object at rest will stay at rest,

and an object in motion will stay in

motion at constant velocity, unless acted

upon by an unbalanced force.

2nd Law – Force equals mass times

acceleration.

3rd Law – For every action there is an

equal and opposite reaction.

4.
1st Law of Motion

(Law of Inertia)

An object at rest will stay at

rest, and an object in motion

will stay in motion at

constant velocity, unless acted

upon by an unbalanced force.

(Law of Inertia)

An object at rest will stay at

rest, and an object in motion

will stay in motion at

constant velocity, unless acted

upon by an unbalanced force.

5.
1 Law

st

Inertia is the

tendency of an

object to resist

changes in its

velocity:

whether in

motion or

These pumpkins will not move unless acted on

motionless. by an unbalanced force.

st

Inertia is the

tendency of an

object to resist

changes in its

velocity:

whether in

motion or

These pumpkins will not move unless acted on

motionless. by an unbalanced force.

6.
1 Law

st

Once airborne,

unless acted on

by an

unbalanced force

(gravity and air –

fluid friction), it

would never

stop!

st

Once airborne,

unless acted on

by an

unbalanced force

(gravity and air –

fluid friction), it

would never

stop!

7.
1 Law

Unless acted

upon by an

unbalanced

force, this golf

ball would sit on

the tee forever.

Unless acted

upon by an

unbalanced

force, this golf

ball would sit on

the tee forever.

8.
Why then, do we observe

every day objects in motion

slowing down and becoming

motionless seemingly without an

outside force?

It’s a force we sometimes cannot see –

friction.

every day objects in motion

slowing down and becoming

motionless seemingly without an

outside force?

It’s a force we sometimes cannot see –

friction.

9.
Objects on earth, unlike the

frictionless space the moon

travels through, are under the

influence of friction.

frictionless space the moon

travels through, are under the

influence of friction.

10.
What is this unbalanced force that acts on an

object in motion?

There are four main types of friction:

Sliding friction: ice skating

Rolling friction: bowling

Fluid friction (air or liquid): air or water resistance

Static friction: initial friction when moving an object

object in motion?

There are four main types of friction:

Sliding friction: ice skating

Rolling friction: bowling

Fluid friction (air or liquid): air or water resistance

Static friction: initial friction when moving an object

11.
Slide a book

across a table and

watch it slide to a rest

position. The book

comes to a rest

because of the

presence of a force -

that force being the

force of friction -

which brings the book

to a rest position.

across a table and

watch it slide to a rest

position. The book

comes to a rest

because of the

presence of a force -

that force being the

force of friction -

which brings the book

to a rest position.

12.
In the absence of a force of friction, the book

would continue in motion with the same speed

and direction - forever! (Or at least to the end

of the table top.)

would continue in motion with the same speed

and direction - forever! (Or at least to the end

of the table top.)

13.
Newtons’s 1 Law and You

st

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

Because of inertia, objects (including you)

resist changes in their motion. When the

car going 80 km/hour is stopped by the

brick wall, your body keeps moving at 80

st

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

Because of inertia, objects (including you)

resist changes in their motion. When the

car going 80 km/hour is stopped by the

brick wall, your body keeps moving at 80

14.
2 Law

15.
2 Law

nd

The net force of an object is

equal to the product of its mass

and acceleration, or F=ma.

nd

The net force of an object is

equal to the product of its mass

and acceleration, or F=ma.

16.
2 Law

nd

When mass is in kilograms and acceleration is

in m/s/s, the unit of force is in newtons (N).

One newton is equal to the force required to

accelerate one kilogram of mass at one

meter/second/second.

nd

When mass is in kilograms and acceleration is

in m/s/s, the unit of force is in newtons (N).

One newton is equal to the force required to

accelerate one kilogram of mass at one

meter/second/second.

17.
2 Law (F = m x a)

nd

How much force is needed to accelerate a 1400

kilogram car 2 meters per second/per second?

Write the formula

F=mxa

Fill in given numbers and units

F = 1400 kg x 2 meters per second/second

Solve for the unknown

2800 kg-meters/second/second or 2800 N

nd

How much force is needed to accelerate a 1400

kilogram car 2 meters per second/per second?

Write the formula

F=mxa

Fill in given numbers and units

F = 1400 kg x 2 meters per second/second

Solve for the unknown

2800 kg-meters/second/second or 2800 N

18.
If mass remains constant, doubling the acceleration, doubles the force. If force remains

constant, doubling the mass, halves the acceleration.

constant, doubling the mass, halves the acceleration.

19.
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 = ma F = ma

98 N = 10 kg x 9.8 m/s/s 9.8 N = 1 kg x 9.8

m/s/s

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 = ma F = ma

98 N = 10 kg x 9.8 m/s/s 9.8 N = 1 kg x 9.8

m/s/s

20.

21.
Check Your Understanding

1. What acceleration will result when a 12 N net force 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 of 5 m/s 2.

Determine the mass.

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

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

m/sec/sec?

1. What acceleration will result when a 12 N net force 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 of 5 m/s 2.

Determine the mass.

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

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

m/sec/sec?

22.
Check Your Understanding

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/s

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

mass.

16 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?

66 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?

9800 kg-m/sec/sec or 9800 N

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/s

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

mass.

16 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?

66 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?

9800 kg-m/sec/sec or 9800 N

23.

24.
3 Law

rd

For every action, there is an

equal and opposite reaction.

rd

For every action, there is an

equal and opposite reaction.

25.
3 Law

According to Newton,

whenever objects A and

B interact with each

other, they exert forces

upon each other. When

you sit in your chair,

your body exerts a

downward force on the

chair and the chair

exerts an upward force

on your body.

According to Newton,

whenever objects A and

B interact with each

other, they exert forces

upon each other. When

you sit in your chair,

your body exerts a

downward force on the

chair and the chair

exerts an upward force

on your body.

26.
3 Law

rd

There are two forces

resulting from this

interaction - a force on

the chair and a force on

your body. These two

forces are called action

and reaction forces.

rd

There are two forces

resulting from this

interaction - a force on

the chair and a force on

your body. These two

forces are called action

and reaction forces.

27.
Newton’s 3rd Law in Nature

Consider the propulsion of a

fish through the water. A fish

uses its fins to push water

backwards. In turn, the water

reacts by pushing the fish

forwards, propelling the fish

through the water.

The size of the force on the

water equals the size of the

force on the fish; the direction

of the force on the water

(backwards) is opposite the

direction of the force on the

fish (forwards).

Consider the propulsion of a

fish through the water. A fish

uses its fins to push water

backwards. In turn, the water

reacts by pushing the fish

forwards, propelling the fish

through the water.

The size of the force on the

water equals the size of the

force on the fish; the direction

of the force on the water

(backwards) is opposite the

direction of the force on the

fish (forwards).

28.
3 Law

Flying gracefully

through the air,

birds depend on

Newton’s third

law of motion. As

the birds push

down on the air

with their wings,

the air pushes

their wings up

Flying gracefully

through the air,

birds depend on

Newton’s third

law of motion. As

the birds push

down on the air

with their wings,

the air pushes

their wings up

29.
Consider the flying motion of birds. A bird flies by

use of its wings. The wings of a bird push air

downwards. In turn, the air reacts by pushing the bird

upwards.

The size of the force on the air equals the size of the

force on the bird; the direction of the force on the air

(downwards) is opposite the direction of the force on

the bird (upwards).

Action-reaction force pairs make it possible for birds

to fly.

use of its wings. The wings of a bird push air

downwards. In turn, the air reacts by pushing the bird

upwards.

The size of the force on the air equals the size of the

force on the bird; the direction of the force on the air

(downwards) is opposite the direction of the force on

the bird (upwards).

Action-reaction force pairs make it possible for birds

to fly.

30.

31.
Other examples of Newton’s

Third Law

The baseball forces the

bat to the left (an

action); the bat forces

the ball to the right (the

reaction).

Third Law

The baseball forces the

bat to the left (an

action); the bat forces

the ball to the right (the

reaction).

32.
3 Law

rd

Consider the motion of

a car on the way to

school. A car is

equipped with wheels

which spin backwards.

As the wheels spin

backwards, they grip the

road and push the road

backwards.

rd

Consider the motion of

a car on the way to

school. A car is

equipped with wheels

which spin backwards.

As the wheels spin

backwards, they grip the

road and push the road

backwards.

33.
3 Law

The reaction of a rocket is

an application of the third

law of motion. Various

fuels are burned in the

engine, producing hot

gases.

The hot gases push against

the inside tube of the rocket

and escape out the bottom

of the tube. As the gases

move downward, the rocket

moves in the opposite

direction.

The reaction of a rocket is

an application of the third

law of motion. Various

fuels are burned in the

engine, producing hot

gases.

The hot gases push against

the inside tube of the rocket

and escape out the bottom

of the tube. As the gases

move downward, the rocket

moves in the opposite

direction.