Download Chapter 3 Newton`s First Law

Part I: Mechanics
7-Jul-17
Physics 1 (Garcia) SJSU
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Aristotle was great
philosopher but not
such a good scientist.
Aristotle’s theory of
motion is wrong.
Took 2000 years
before Galileo got
motion right.
7-Jul-17
Physics 1 (Garcia) SJSU
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Every object has a “natural” state.
In “natural motion”, “Earth”
elements (stone, apple, you, etc.)
are drawn to the Earth.
Heavier objects are more strongly
attracted so they fall faster (stone
falls faster than a feather).
Aristotle
Important: These are Aristotle’s ideas, but he’s wrong!
Reality
7-Jul-17
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Pushing or pulling an object causes
“unnatural” motion (or “violent” motion).
If cause is removed (stop pushing) then
object returns to “natural” state and stops
moving.
Pushed brick slides but then comes to a stop
BRICK
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BRICK
Aristotle thought objects in
unnatural motion would first
stop, then fall.
He was wrong!
“Unnatural”
“Natural”
Reality
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Pull of gravity (weight)
obviously larger for a
bowling ball than for a
baseball.
YET the two balls fall
with the same speed.
What about a feather?
(Answer in Chapter 4)
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Downhill: Speed increases
Uphill: Speed decreases
Flat surface: Speed increases, decreases,
or constant?
Questions existence of “natural Earth” state of not moving.
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Imagine ball rolling down hill
then back uphill
Again, questions existence of “natural Earth” state of not moving.
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An object at rest remains at rest &
an object in motion remains
in uniform motion*,
unless a force acts on the object.
*Moving in a straight line with constant speed.
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Object at rest
(not moving)
then
then
No force on
object
Uniform motion
Note: Object at rest is just uniform motion at zero speed.
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When a moving
train stops, you
continue moving
forward.
When the stopped
train starts moving
again, you remain
stationary and are
thrown backwards
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All moving together
WALL
Bottom car stops.
Other two continue
With same speed.
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Flower
Vase
Yank quickly
Tablecloth
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Upper String
Heavy
Cannon
Balls
Which string
breaks in each
case?
Lower String
Slow
pull
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Quick
yank
What happens
in each case?
Which roll has
more inertia?
Quick
yank
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When several forces act on an object, the forces
add together.
Sum of forces called net force or total force
3 Newtons
5 Newtons
8 Newtons
BRICK
same as
The Newton is metric unit of force (about 1/5 pound).
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?
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If an object is at rest then the net force must be
zero. Similarly if in uniform motion.
Zero Newtons
(No Force)
3 Newtons
3 Newtons
BRICK
same as
When this happens we say that forces “balance.”
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Scale reads
5 Newtons
5+7=10+2
Scale reads
7 Newtons
STICK (2 Newtons)
Stick is at rest.
10 Newton
Iron mass
The two upward forces (by people holding the scales) must always be
equal to the two downward forces (weight of stick and iron mass).
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Solid surfaces exert a force, called a support
force, on objects pressed against them.
100 Newton
Gold Brick
100 Newton
Support force
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Downward force (weight)
balanced by upward force
(support).
How much is the
net force on the brick?