Newton’s First Law of Motion

Inertia

History of the motion concept

ARISTOTLE (384 – 322 B.C)

Famous philosopher, scientist and educator Tutored Alexander the Great

Aristotle divided motion into 2 types - Natural motion - Violent motion

Natural Motion

This type of motion was thought to be straight down or straight up and not caused by force

ex: A boulder falling to the ground

or smoke rising up in the air.

This came from the naturalconcept that “heavy thingsFall” and “light things rise”

What about the heavens?

Aristotle said that circular motion was natural

Planets and stars moved in perfect circles around the EARTH.

The EARTH was the

centre of the universe and DID NOT MOVE!!!

Violent Motion

This was imposed motion caused by forces that pushed or pulled on an object

Ex: A cart moved because it was pulled by a horse

Violent motion always had an external cause

In comes Copernicus (1473-1543)

The simplest way to interpret astronomical observations was to assume that Earth and other planets move around the Sun.

At that time, this very controversial as people believed that the Earth was the centre of the universe.

So to escape persecution, Copernicus worked in secret and published his ideas just before he died.

Then comes Galileo

Galileo was known for the first telescope he built.

After 15 years he published his views and was persecuted by the church and sentenced to house arrest.

His greatest contribution to Physics was demolishing the notion that a force is necessary to keep an object in motion.

He damaged his eyes by looking at the sun and died blind

Friction

Friction is a force that acts between materials that touch as they move past each other.

It is caused by irregularities in the surfaces of objects that are touching.

If friction were absent, a moving object would need no force to remain in motion

Galileo’s test

Slope is downwards and speed is increasing

Slope is upwards and speed is decreasing

What happens to a ball rolling on a level plane with 0 friction?

The ball would roll with constant velocity

If friction were absent, the ball would roll forever

Galileo’s second experiment

Rolling ball experiment

What is the final position of the ball – A, B or C ?

The ball reaches the same height from which it was droppedA

B

C

Finally

If there is no friction, the ball would never stop unless it hits something

Where is the final position?

INERTIA

The ball tried to resist changes to its state of motion.

The property of a body to resist changes to its state of motion is called INERTIA

Due to inertia a body tries to travel in a straight line path forever

Think about this now

How about this one?

ANS: B

One more . . .

ANS: D

Next

Take a text book

Work silently on the worksheet.

Question 17 has 2 answers each for A, B and C

Return your textbook to the bookcase and turn in your worksheet to the teacher.

Newton’s First Law of Motion

Inertia