Chapter 3: Newton’s 1 st Law
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Transcript of Chapter 3: Newton’s 1 st Law
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?