Chapter 4Newton’s First Law of Motion

The Law of Inertia

4.1 Aristotle on Motion

• Aristotle said there are 2 types of motion:1. Natural motionNatural motion – motion straight up and straight down; objects seek natural resting places2. Violent motionViolent motion – imposed motion; objects are set in motion by forces

4.2 Copernicus and the Moving Earth

• Aristotle believed the Sun revolves around the Earth because the Earth does not appear to be moving• This view was accepted by

society for many centuries.

• Copernicus later put forward the theory that the Earth actually rotates around the Sun.• Galileo, a contemporary of

Copernicus, agreed with him based on his telescopic observations• Galileo was persecuted for these

views.

4.3 Galileo on Motion

• Force – any push or pull• Friction – the name given to

the force that acts between materials that touch as they move past each other

FrictionWithout friction, an object would continue moving forever on a level surface. Once the object is in motion, no force is needed for the object to continue moving.

4.3 Galileo on Motion

• Galileo believed the Earth IS in constant motion and that the Earth revolves around the Sun.• He believed that once set in motion,

an object will continue to move by itself and that it does not need a force to continue moving.

Figure 4.2• Objects rolling downhill tend to

speed up (accelerate)• Objects rolling uphill tend to slow

down (decelerate)• What about those on an even

surface?

Inertia• Naturally, the ball would keep going

in the same direction.

• INERTIA – INERTIA – a resistance to change in the state of motion

Inertia

• Think about what happens when you suddenly stop a car.

• What does your body do?

Inertia

Inertia

Your body continues to move in the same direction.

You are RESISTING change in your state of motion:

INERTIA!

4.4Newton’s First Law of Motion

• Isaac Newton was born soon after Galileo died.• In 1665, at the age of 23, Newton

stated his 3 Laws of Motion which we still study today.

Newton’s First Law of Motion(Law of Inertia)

• “Every object continues in its state of rest, or of motion in a straight line at constant speed, unless it is compelled to change that state by forces exerted upon it”.

4.5 Mass – A measure of inertia

• MASS – MASS – the amount of material present in an object• The amount of INERTIA an

object has is dependent on its mass.

Mass, Volume, and Weight

• Volume is the measure of space an object takes up.• Weight is the measure of

gravitational attraction an object has to Earth.• Mass is notnot the same as weight!!

Force• Force is measured in NEWTONSNEWTONS9.8 Newtons (N) of force = 1 kilogram or,

1 kg = 9.8 N (on the planet Earth)Remember, gravity is a downward FORCE!

Example• What is the weight (in Newtons)

of a 10 kg bowling ball?

• Answer:10 kg X 9.8 N/1 kg = 98 N

4.6 Net Force

Definition:The combination of all the forces that act on an object is the NET FORCE.

(Figure 4.10 page 51)

4.7 Equilibrium – When Net Force Equals Zero

• A book is laying on the table• What forces are acting on the

book?

4.7 Equilibrium – When Net Force Equals Zero

• Support Force (Normal Force) – the force that balances the force of an object on the surface–(the table’s force pushing back

on the book)–Normal force is equal to the

weight of the book

4.7 Equilibrium – When Net Force Equals Zero

• When an object is at rest, with the net force on it being zero, we say it is in a state of

EQUILIBRIUM

Ch. 4 Vocabulary

• EQUILIBRIUM• FORCE• FRICTION• INERTIA• LAW OF

INERTIA• WEIGHT

• NEWTON’S 1ST LAW• NET FORCE• KILOGRAM• MASS• SUPPORT

FORCE• NEWTON