Chapter Six: Laws of Motion

6.1 Newton’s First Law

6.2 Newton’s Second Law

6.3 Newton’s Third Law and

Momentum

6.1 Force changes motion

A force is a push or pull, or any

action that is able to change motion.

6.1 Law of inertia

Newton’s first law says that objects

continue the motion they already

have unless they are acted on by a

net force.

If the net force is zero, an object at

rest will stay at rest.

If an object is acted upon by

unbalanced forces, its motion will

change.

6.1 Net force

Newton’s first law is

often written in

terms of the net

force:

“An object at rest

will stay at rest and

an object in motion

will continue in

motion at constant

velocity UNLESS

there is a net force.”

According to these vectors, in

what direction is the net force?

6.1 Force changes motion

Forces can be used to increase or

decrease the speed of an object, or

to change the direction an object is

moving.

6.1 Law of inertia

Inertia is the

property of an

object that resists

changes in motion.

Objects with more

mass have more

inertia and are

more resistant to

changes in their

motion.Which ball has more

inertia?

A car drives along the highway at constant

velocity. Find the car’s weight and the

friction force if the engine produces a

force of 2,000 newtons between the tires

and the road and the normal force on the

car is 12,000 N.

Solving Problems

1. Looking for:

…weight of car in newtons, force due to

friction

2. Given:

…ForceN

= 12,000N (up);

…ForceE

= 2,000N (forward)

3. Relationships:

Newton’s 1st

Law:

net force = zero at constant velocity; so

ForceN

= ForceW

and ForceE

= ForceF

Solving Problems

FE = 200 N

4. Solution

Draw a free body

diagram.

There is no net force

upward, so the weight of

the car is an equal

downward force of

−12,000 N.

The forward engine force

balances the friction

force so the friction

force is −2,000 N.

Solving Problems

FF = -200 N

FW

= -

12,0

00N

FN

= 1

2,0

00N

6.2 Newton’s second law

Newton’s first law tells us that

motion cannot change without a

net force.

According to Newton’s second law,

the amount of acceleration

depends on both the force and the

mass.

6.2 The newton

The S.I. unit of

force (newton) is

defined by the

second law.

A newton is the

amount of force

needed to

accelerate a 1 kg

object by 1m/s.

6.2 Newton’s second law

There are three main ideas related

to Newton’s Second Law:

1. Acceleration is the result of

unbalanced forces.

2. A larger force makes a

proportionally larger acceleration.

3. Acceleration is inversely

proportional to mass.

6.2 Newton’s second law

Unbalanced forces cause changes in

speed, direction, or both.

6.2 Acceleration and force

The second law says

that acceleration is

proportional to force.

If force is increased

or decreased,

acceleration will be

increased or

decreased by the

same factor.

6.2 Acceleration and direction

Another important factor of the second law

is that the acceleration is always in the

same direction as the net force.

6.2 Acceleration and mass

The greater the mass, the smaller the

acceleration for a given force.

This means acceleration is inversely

proportional to mass.

6.2 Acceleration, force and mass

The acceleration caused by a force is

proportional to force and inversely

proportional to mass.

The stronger the

force on an object,

the greater its

acceleration.

Force is directly

proportional to

acceleration.

If twice the force

is applied, the

acceleration is

twice as great.

The greater the

mass, the smaller

the acceleration

for a given force.

Mass is

inversely

related to force.

An object with

twice the mass

will have half

the acceleration

if the same

force is applied.

6.2 Applying the second law

Keep the following

important ideas in mind:

1. The net force is what

causes acceleration.

2. If there is no acceleration,

the net force must be

zero.

3. If there is acceleration,

there must also be a net

force.

4. The force unit of newtons

is based on kilograms,

meters, and seconds.

A car has a mass of 1,000 kilograms.

If a net force of 2,000 N is exerted

on the car, what is its acceleration?

1. Looking for:

…car’s acceleration

2. Given

…mass = 1,000 kg; net force = 2,000 N

3. Relationships:

a = F / m

4. Solution:

2, 000 N ÷ 1,000 kg = 2 N/kg = 2 m/s2

Solving Problems

6.3 Newton’s Third Law

Newton’s Third

Law (action-

reaction) applies

when a force is

placed on any

object, such as a

basketball.

6.3 The Third Law: Action/Reaction

Newton’s Third Law

states that every action

force creates a reaction

force that is equal in

strength and opposite

in direction.

There can never be a

single force, alone,

without its action-

reaction partner.

6.3 Action and reaction

When sorting out

action and

reaction forces it

is helpful to

examine or draw

diagrams.

Here the action force is on the ________________, and

the reaction force is on the _______________.

Solving Problems

A woman with a

weight of 500

newtons is sitting

on a chair.

Describe one

action-reaction pair

of forces in this

situation.

1. Looking for:

…pair of action-reaction forces

2. Given

…girl’s forceW

= -500 N (down)

3. Relationships:

Action-reaction forces are equal and opposite and act on

different objects.

4. Solution

Draw a free body diagram

The downward force of 500 N exerted by the woman on

the chair is an action.

Therefore, the chair acting on the woman provides an

upward force of 500 N and is the reaction.

Solving Problems

Fw = -500 N

Fc = 500 N

6.3 Collisions

Newton’s third law tells us that any time

two objects hit each other, they exert

equal and opposite forces on each other.

The effect of the force is not always the

same.

6.3 MomentumMomentum is the mass of a object

times its velocity.

The units for momentum are

kilogram-meter per second (kg·m/s).

6.3 MomentumThe law of

conservation of

momentum states

that as long as the

interacting objects

are not influenced

by outside forces

(like friction) the

total amount of

momentum is

constant or does

not change.

6.3 Momentum

The result of a

skateboarder

throwing a 1-kg ball at

a speed of -20 m/sec

is that he and the

skateboard with a

total mass of 40 kg

move backward at a

speed of +0.5 m/sec

(if you ignore friction).

We use positive and

negative numbers to show

opposite directions.

6.3 Collisions

When a large truck

hits a small car, the

forces are equal.

The small car

experiences a much

greater change in

velocity much more

rapidly than the big

truck. Which vehicle ends up

with more damage?

Solving Problems

If an astronaut in

space were to release

a 2-kilogram wrench

at a speed of 10 m/s,

the astronaut would

move backward at

what speed?

The astronaut’s mass

is 100 kilograms.

1. Looking for:

… the velocity of the astronaut (backward)

2. Given

…velocity1

= 10 m/s; mass1= 2 kg;

...mass2

= 100 kg;

3. Relationships:

m1v

1= m

2v

2

4. Solution

Draw a free body diagram.

Solving Problems