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Quick Quiz
Did you
read the
chapter?!
Which of the following is going to be
influenced by Strong interactions?
„
A.
B.
C.
D.
E.
galaxy
quark
solar system
person
molecule
(A) Yes
(B) No
1
Main Ideas
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natural motion: how things move in the absence
of external forces
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Newton’s Second Law of Motion
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Force
Newton’s First Law of Motion
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2
how an object moves in response to an applied
force
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Newton’s Third Law of Motion
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a push or pull on one object by another
anything that causes a change in
motion
measured in Newtons (pounds)
Has a specific direction
Has nothing to do with motion
Tells where forces come from
3
Net Force
Identifying forces
The sum of all the forces acting on an
object.
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Fundamental forces –
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Also called the “resultant force” or total
force.
Forces must be added as vectors
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4
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2 north + 2 south = 0
2 north + 2 east = √8 north east
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5
PS100, Sect. 3, Winter 2000
Does the object in question have mass?
Does the object in question have charge?
Contact forces –
„
can act at a distance.
anything touching our object
Normal forces
Friction
Tension
6
Chapter 3, Page 1
Newton’s First Law:
„
When does 1st law apply?
Unless acted on by a net external force,
an object will continue in motion at a
uniform speed and in a uniform
direction
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If the total force is zero.
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No net force Æ No change in motion.
If there are no forces
If the forces balance
If the object is at rest.
If the object is moving in a straight line
at constant speed. (Uniform Motion)
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8
Acceleration
Newton’s Second Law
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Force = mass x acceleration
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velocity—both speed and direction of
motion
acceleration--change in a object’s
velocity
Acceleration is any change in speed or
direction.
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10
Mass
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a property of objects that determines
how they respond to a force
different from weight
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Newton’s Second Law
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weight is a force that depends on gravity
objects in space still have mass
How much an object’s motion changes
depends on how hard you push on it, and
how big the object is.
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stronger forces cause more acceleration
larger mass results in less acceleration
acceleration is in the direction of the net force
measured in grams or kilograms (slugs)
11
PS100, Sect. 3, Winter 2000
12
Chapter 3, Page 2
When does
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„
Sally Discovers
Newton’s Third Law
law apply
Any time there is a net force.
Any time the motion of an object is
changing
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2nd
Change in speed
Change in direction
Note an object is always either in
accelerated or in uniform motion, so
either the 2nd law or 1st law applies.
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14
Newton’s Third Law
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When do you use 3rd law
forces come from two interacting
objects (objects cannot act on
themselves)
there are forces on both objects
these forces are equal in magnitude
and opposite in direction
Rules:
1. Just because two forces are equal and
opposite does NOT mean they are an
action-reaction pair.
¾
An action-reaction pair acts on a single pair of
objects whose roles switch.
2. The 3rd law of motion doesn’t have anything
to do with motion.
¾
A 3rd law pair measures the strength of a single
interaction, NOT net force.
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Newton’s Third Law Examples
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firing a shotgun
woman standing on the floor
bouncing a basketball
playing a guitar
rock climbing (“pull yourself up?”)
drag racing
1.
2.
3.
17
PS100, Sect. 3, Winter 2000
Demo Questions
Identify the forces
Decide whether the 1st or 2nd law applies
Decide if the 3rd law is pertinent
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Chapter 3, Page 3
Summary
Cartoon Laws of Physics
Think of some examples of how Newton’s
Laws of Motion would be different if
written by cartoon animators (Road
Runner cartoons, for instance).
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19
PS100, Sect. 3, Winter 2000
Newton’s Laws
Force
Mass
Acceleration
20
Chapter 3, Page 4