Transcript Inertia And Force Diagrams

“The Force”
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“An energy field created by all living things. It
surrounds us, penetrates us, and binds the galaxy
together.”
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The Force has two components:
Light side
 Dark side
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The Real Force
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Something that causes an object’s motion to
change (causes acceleration).
A “push” or a “pull.”
Common Examples of forces: Gravity fields,
pushing on something, compressing a spring, a
magnetic field, tension, friction, and the
“normal” force.
Units are Newtons (N)
First Law of Motion: “The Law of Inertia”
An object at rest remains at rest, and an object in motion
continues in motion with constant velocity unless the
object experiences a net external force.
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What does this law tell us?
• Objects in equilibrium do not accelerate. Static equilibrium
(rest) and equilibrium (constant velocity) are both the result
of an object with zero net force.
Galileo’s Unique Idea
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Objects don’t need a force to keep moving!
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Every object naturally wants to maintain its state of
motion or rest
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INERTIA! (resistance to change in motion)
Refined by Newton in 1800’s:
Basic Info: Inertia
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Inertia depends on:
Mass
 Shape/Mass Distribution of object- rotational inertia
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Solid Cylinder (like a wheel of cheese,)
 Hoop (like a bicycle tire)
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Inertia does NOT depend on:
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Velocity/Speed of object
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It takes the same amount of force to speed a bus up as to slow it
down!
The Definition of Force
“If you insist upon a precise definition of force, you will never get it!” - Richard Feynmann
Forces are not directly observable, but the effect of force is perceived .
Newton’s Second Law defines force.
• A newton is defined as the force required to accelerate one
kilogram of mass at a rate of one meter per second squared.
 meter 
1 newton  1 kilogram    1
 second 2 
• A newton is the metric equivalent of the pound.
Both are units of force, not mass.
• A newton converts to a little less
than a quarter pound.
1 newton  0.225 pound
1 pound = 4.45 newton
Mass versus Weight
Mass
Mass is classical defined as an amount or quantity of
matter. The modern definition is the amount of inertia
object possesses.
Mass is universal; it doesn’t depend on location.
Weight
Weight is defined as a force caused by gravity acting on a mass.
Weight is local; it depends on gravity.
weight = mass  gravity
Fg  mg
mass
force
Metric
kilogram
newton
British
slug
pound
Normal Force, Tension, and Applied Force
Normal Force, Fn
A contact force, often called a support
force, that acts perpendicular to the
surfaces in contact.
Normal means perpendicular.
On a level surface, the normal force
will balance the weight of an object, as
long as no other forces act vertically.
Fn
PHYSICS
Fg = mg
Tension, FT
A pulling force in strings,
PHYSICS
ropes, cables, etc.
Tension force always pulls away from a mass
(opposite of compression).
Applied Force, Fa
An applied force is any external force.
rope
FT
Fa
PHYSICS
Friction Forces
Friction arises from molecular bonding between surfaces
A contact force that always acts parallel to the surfaces in
contact, and always opposes motion.
Ff
PHYSICS
book pulled
Fa
Ff
wheel driven
Ff
velocity
PHYSICS
book dragged
Fa
Basic Info: Force Diagrams
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Definition: A Diagram that shows all the forces
acting on a body
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Does NOT include forces exerted by the body!
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Forces are drawn as vectors.
Free Body Diagram
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Simple drawing of all forces
working on an object or system
Use a box or dot to represent
the object or system
All forces move away from the
box.
The size of the arrow needs to
be proportional to the size of
the force.
Remember: gravity will always
affect an object and so Fg will
always be in a F.B.D!
Free Body Diagrams
A free body diagram identifies all action forces on an
object so that the resultant force can be determined.
Balanced Forces
When the sum of all forces is equal
to zero the object does not accelerate
(at rest or constant velocity).
Fs
Fn
PHYSICS
v
F  0
Fg
Unbalanced Forces
When the sum of all forces is not
equal to zero, the object accelerates
in the direction of the resultant force.
v
v
F  ma
Fa
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Fk
Fn
PHYSICS
acceleration
Fg
Fa