CHAPTER 3: NEWTON’S 1ST LAWInertia

NEWTON’S FIRST LAW

If a ball was lying in the middle of an empty field and began moving, what would be your first thought?

We do not believe that changes in motion occur without cause

ARISTOTLE

Greek scientist studied motion and divided it into two types Natural Motion

It was natural for heavy objects to fall and for light things to rise (such as smoke)

Circular motion was natural for the heavens, thus stars and planets revolved

Violent Motion Imposed motion to objects (a cart pulled by a

horse) Objects in a resting place must have an external

cause for movement

COPERNICUS

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

Very controversial Had to work on his ideas in secret to escape

persecution First copy of his work reached him on the day of

his death

GALILEO

Galileo-foremost scientist of the time arrested for support of Copernicus

Had to do all his work without mechanical clocks Used pulse, dripping of water

droplets, etc Einstein called him the father of

modern physics Galileo demolished the notion

that a force is necessary to keep an object moving.

FRICTION

Friction is the name given to the force that acts between materials that touch as the move past each other. Caused by the irregularities in the surfaces of

objects Even very smooth surfaces have microscopic

irregularities that obstruct motion Galileo argued that only when friction is

present is a force needed to keep an object moving.

FRICTION

Galileo tested this theory by rolling balls along plane surfaces tilted at different angles.

a. The ball rolling down the incline rolls up the opposite incline and reaches initial height.

b. As the angle of the upward incline is reduced, the ball rolls a greater distance before reaching initial height

c. If there is no friction, the ball will never stop rolling unless it hits something

FRICTION

Aristotle had claimed that the nature of the ball would be to be at rest.

Galileo claimed that a moving body tends to keep moving and that it is natural. Every material object resists change to its state of motion.

Inertia is the property of a body to resist changes to its state of motion.

NEWTON’S LAW OF INERTIA

By age 24, Isaac Newton had developed his famous laws of motion

Newton’s first law, law of inertia, states “Every object continues in a state of rest, or of uniform speed in a straight line, unless acted on by a nonzero net force.”

Restate this in your own terms. What examples of this did you observe? Relate this to the equilibrium rule

OYB

What is Newton’s First Law of Motion?

INERTIA

What determines the amount of inertia? Ever heard the phrase, “The bigger they are,

the harder they fall?” The more mass an object has, the greater its

inertia and the more force it takes to change its state of motion. Mass is a measure of the inertia of an object. Do not confuse mass and volume

MASS

Mass is not the same as weight How do we normally determine the amount

of matter in an object? Mass is more fundamental than weight.

Mass is the amount of material in an object: depends only on the number of and kind of atoms

Weight is the measure of the gravitational force acting on an object.

MASS VS. WEIGHT

What is mass? Quantity of matter in an object A measure of the inertia or “laziness” that an

object exhibits in response to any effort made to start, stop, or change its state of motion

What is weight? Force of gravity on an object

They are not the same, but are proportional in a given place. In the same location, twice the mass weighs twice as much.

OYB

Does a 2-kilogram bunch of bananas have twice as much inertia as a 1-kilogram loaf of

bread?Twice as much mass?

Twice as much volume?Twice as much weight, when weighed in the same location?

(Write key word: then yes or no)

THE MOVING EARTH

Copernicus introduced the idea of a moving earth in the 16th century

His theory would have to mean that the Earth has to move at a speed of 107,000 km/hr to circle the sun in one year. OYB Convert this speed to km/sec

Argument against: a bird at rest in a tree sees a fat worm on the earth below him. The bird drops down vertically and catches it. This can’t be possible if the Earth moves as suggested because the worm would have been swept away by the Earth for a distance of 30 km. This is proof that Earth must be at rest.

OBJECTS MOVE WITH EARTH

Can you refute the previous argument? Invoke the idea of inertia

Objects in motion remain in motion if no unbalanced forces act upon them

What all is moving? Earth, tree, branch of tree, bird, worm, air between

When the bird drops from the branch, its initial sideways motion remains 30 km/s and is unaffected by the motion of the total environment

The speed of Earth is relative to the sun, not of the bird to the worm

OYB

How fast are you moving relative to Earth when you are standing still?

How fast are you moving relative to the sun?

OYB

If you’re in a smooth riding bus that is going at 40 km/hr and you flip

a coin vertically, how fast does the coin move

horizontally while in midair?

OBJECTS MOVE WITH VEHICLES Why was it so hard for people then to accept

ideas like this? Didn’t know about inertia Not accustomed to moving in high speed

vehicles Slow, bumpy rides in carriages do not lend themselves

to experiments revealing inertia Flip a coin in a car or airplane, what happens?

Aristotle didn’t recognize inertia because he didn’t realize that all moving things follow the same rules

INERTIA SAFETY

In Taipei, there is a more than 3 million kg steel ball that helps stabilize the 101 story building against vibrations caused by earthquakes and strong winds.

Why would this work?