14-May-15 Physics 1 (Garcia) SJSU Chapter 2 Newton’s First Law of Motion Part I: Mechanics.
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Transcript of 14-May-15 Physics 1 (Garcia) SJSU Chapter 2 Newton’s First Law of Motion Part I: Mechanics.
Apr 18, 2023 Physics 1 (Garcia) SJSU
Chapter 2 Newton’s First Law
of Motion
Part I: Mechanics
Apr 18, 2023 Physics 1 (Garcia) SJSU
Aristotle & Galileo
• 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.
Apr 18, 2023 Physics 1 (Garcia) SJSU
Motion according to Aristotle (I)
• 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
Reality
Important: These are Aristotle’s ideas, but he’s wrong!
Apr 18, 2023 Physics 1 (Garcia) SJSU
Motion according to Aristotle (II)
• 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.
BRICK BRICK
Pushed brick slides but then comes to a stop
Apr 18, 2023 Physics 1 (Garcia) SJSU
Natural & Unnatural MotionAristotle thought objects in
unnatural motion would first stop, then fall.
He was wrong!
“Unnatural”
“Natural”Reality
Apr 18, 2023 Physics 1 (Garcia) SJSU
Galileo & Falling
• 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)
Apr 18, 2023 Physics 1 (Garcia) SJSU
Galileo’s Inclines (I)
Downhill: Speed increases
Uphill: Speed decreases
Flat surface: Speed increases, decreases,or constant?
Questions existence of “natural Earth” state of not moving.
Apr 18, 2023 Physics 1 (Garcia) SJSU
Galileo’s Inclines (II)
Again, questions existence of “natural Earth” state of not moving.
Imagine ball rolling down hill then back uphill
Apr 18, 2023 Physics 1 (Garcia) SJSU
Newton’s First Law of Motion
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.
Apr 18, 2023 Physics 1 (Garcia) SJSU
Newton’s First Law of Motion
Object at rest
(not moving)No force on
object
Uniform motion
Note: Object at rest is just uniform motion at zero speed.
then
then
Apr 18, 2023 Physics 1 (Garcia) SJSU
Demo: Riding Light Rail
When a moving train stops, you continue moving forward.
When the stopped train starts moving again, you remain stationary and are thrown backwards
Apr 18, 2023 Physics 1 (Garcia) SJSU
Demo: Rolling, rolling, rollingW
ALL
All moving together
Bottom car stops.Other two continueWith same speed.
Apr 18, 2023 Physics 1 (Garcia) SJSU
Demo: Tablecloth Pull
Tablecloth
FlowerVase
Yank quickly
Apr 18, 2023 Physics 1 (Garcia) SJSU
Demo: Inertia Balls
HeavyCannonBalls
Upper String
Slowpull
Quickyank
Lower String
Which string breaks in each case?
Apr 18, 2023 Physics 1 (Garcia) SJSU
Demo: Paper Pull
Quickyank
What happens in each case?
Which roll has more inertia?
Apr 18, 2023 Physics 1 (Garcia) SJSU
Net Force
same as
The Newton is metric unit of force (about 1/5 pound).
When several forces act on an object, the forces add together.
Sum of forces called net force or total force
BRICK
3 Newtons5 Newtons
8 Newtons
Apr 18, 2023 Physics 1 (Garcia) SJSU
Check Yourself
?
Apr 18, 2023 Physics 1 (Garcia) SJSU
Equilibrium Rule
same as
If an object is at rest then the net force must be zero. Similarly if in uniform motion.
BRICK3 Newtons 3 Newtons
Zero Newtons(No Force)
When this happens we say that forces “balance.”
Apr 18, 2023 Physics 1 (Garcia) SJSU
Demo: Up+Up=Down+Down
STICK (2 Newtons)
10 NewtonIron mass
Scale reads5 Newtons
Scale reads7 Newtons
The two upward forces (by people holding the scales) must always be equal to the two downward forces (weight of stick and iron mass).
Stick is at rest.
5+7=10+2
Apr 18, 2023 Physics 1 (Garcia) SJSU
Support Force
Solid surfaces exert a force, called a support force, on objects pressed against them.
100 NewtonGold Brick
100 NewtonSupport force
Downward force (weight) balanced by upward force (support).
How much is the net force on the brick?