1. Chapter 2 Newton’s First Law of Motion
Part I: Mechanics
Chapter 2 Newton’s First Law of Motion
15-Apr-17
Physics 1 (Garcia) SJSU

2. 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.
15-Apr-17
Physics 1 (Garcia) SJSU

3. 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
Important: These are Aristotle’s ideas, but he’s wrong!
Reality
15-Apr-17
Physics 1 (Garcia) SJSU

4. 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.
Pushed brick slides but then comes to a stop
BRICK
BRICK
15-Apr-17
Physics 1 (Garcia) SJSU

5. Natural & Unnatural Motion
Aristotle thought objects in unnatural motion would first stop, then fall.
He was wrong!
“Unnatural”
“Natural”
Reality
15-Apr-17
Physics 1 (Garcia) SJSU

6. 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)
15-Apr-17
Physics 1 (Garcia) SJSU

7. 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.
15-Apr-17
Physics 1 (Garcia) SJSU

8. Galileo’s Inclines (II)
Imagine ball rolling down hill then back uphill
Again, questions existence of “natural Earth” state of not moving.
15-Apr-17
Physics 1 (Garcia) SJSU

9. 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.
15-Apr-17
Physics 1 (Garcia) SJSU

10. Newton’s First Law of Motion
Object at rest
(not moving)
then
No force on object
then
Uniform motion
Note: Object at rest is just uniform motion at zero speed.
15-Apr-17
Physics 1 (Garcia) SJSU

11. 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
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Physics 1 (Garcia) SJSU

12. Demo: Rolling, rolling, rolling
WALL
All moving together
Bottom car stops.
Other two continue
With same speed.
15-Apr-17
Physics 1 (Garcia) SJSU

13. Demo: Tablecloth Pull Flower Vase Tablecloth Yank quickly 15-Apr-17
Physics 1 (Garcia) SJSU

14. Demo: Ring Yank Ring Beaker Coin Yank quickly 15-Apr-17
Physics 1 (Garcia) SJSU

15. Demo: Inertia Balls Which string breaks in each case? Upper String
Heavy
Cannon
Balls
Lower String
Slow
pull
Quick
yank
15-Apr-17
Physics 1 (Garcia) SJSU

16. Demo: Paper Pull What happens in each case?
Which roll has more inertia?
Quick
yank
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Physics 1 (Garcia) SJSU

17. Net Force
When several forces act on an object, the forces add together.
Sum of forces called net force or total force
3 Newtons
8 Newtons
BRICK
5 Newtons
same as
The Newton is metric unit of force (about 1/5 pound).
15-Apr-17
Physics 1 (Garcia) SJSU

18. Check Yourself ?
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Physics 1 (Garcia) SJSU

19. Equilibrium Rule
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.”
15-Apr-17
Physics 1 (Garcia) SJSU

20. Demo: Up+Up=Down+Down
Scale reads
5 Newtons
Scale reads
7 Newtons
5+7=10+2
STICK (2 Newtons)
10 Newton
Iron mass
Stick is at rest.
The two upward forces (by people holding the scales) must always be equal to the two downward forces (weight of stick and iron mass).
15-Apr-17
Physics 1 (Garcia) SJSU

21. Support Force
Solid surfaces exert a force, called a support force, on objects pressed against them.
Downward force (weight) balanced by upward force (support).
100 Newton
Gold Brick
100 Newton
Support force
How much is the
net force on the brick?
15-Apr-17
Physics 1 (Garcia) SJSU