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Dynamics
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Dynamics
• Study of the interactions between force and
motion
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Force
• A force is something that is CAPABLE of
causing an object (mass) to change its velocity
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Force
• A force is something that is CAPABLE of
causing an object (mass) to change its velocity
Change in speed
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Force
• A force is something that is CAPABLE of
causing an object (mass) to change its velocity
Change in speed
or
Change in direction
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Force
• A force is something that is CAPABLE of
causing an object (mass) to change its velocity
Change in speed
or
Change in direction
or
Both change direction and speed
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Force
• To be CAPABLE does not mean it must happen
– A force will not always cause a change in velocity
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Force
• To be CAPABLE does not mean it must happen
– A force will not always cause a change in velocity
•
The force must be unbalanced to cause a change
in velocity
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Force
• To be CAPABLE does not mean it must happen
– A force will not always cause a change in velocity
•
The force must be unbalanced to cause a change
in velocity
•
Net Force ≠ zero
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Force
• To be CAPABLE does not mean it must happen
– A force will not always cause a change in velocity
•
The force must be unbalanced to cause a change
in velocity
•
•
Net Force ≠ zero
Net Force = sum of all forces acting on an object
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Force
• To be CAPABLE does not mean it must happen
– A force will not always cause a change in velocity
•
The force must be unbalanced to cause a change
in velocity
•
•
•
Net Force ≠ zero
Net Force = sum of all forces acting on an object
FNet = F1 + F2 + F3 +….. Fn
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Force
• To be CAPABLE does not mean it must happen
– A force will not always cause a change in velocity
•
The force must be unbalanced to cause a change
in velocity
•
•
•
•
Net Force ≠ zero
Net Force = sum of all forces acting on an object
FNet = F1 + F2 + F3 +….. Fn
FNet = ∑ F1-n
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Force
• If the Net Force ≠ zero
– The object being acted upon must accelerate
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4.1 The Concepts of Force and Net Force
This figure illustrates what happens in
the presence of zero and nonzero net
force.
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Force
• When the object exposed to the unbalanced
force begins to accelerate;
 The forces are effectively balanced
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Force
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Force
• When the object exposed to the unbalanced
force begins to accelerate;
 The forces are effectively balanced
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Force
• When the object exposed to the unbalanced
force begins to accelerate;
 The forces are effectively balanced
 The excessive force causes an acceleration in the
direction of the unbalanced force
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Force
• When the object exposed to the unbalanced
force begins to accelerate;
 The forces are effectively balanced
 The excessive force causes an acceleration in the
direction of the unbalanced force
 The acceleration is directly proportional to magnitude
of the unbalanced force
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Types of Forces
• Two broad categories of forces
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Types of Forces
• Two broad categories of forces
• Contact forces
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Types of Forces
• Two broad categories of forces
• Contact forces
• Field forces
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Types of Forces
• Two broad categories of forces
• Contact forces
• Field forces
» forces that act at a distance
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Contact Force
• The surfaces of two separate objects are in contact
• Push
• Pull
• friction
• Tension from a rope, string, etc.
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Field Force
• Force that acts without direct surface contact
• Force acts at a distance
»gravity,
»electric force
»magnetic force
»strong nuclear force
»weak nuclear force
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Inertia
• Inertia is the natural tendency of an object to
maintain its current state of motion
• An object at rest will tend to remain at rest
• An object in motion will tend to maintain its current
state of motion
» Same speed
» Same direction
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Inertia
• Galileo developed the concept of Inertia
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Inertia
• Galileo developed the concept of Inertia
• Inertia is a property of matter that causes it to resist
changes in its velocity
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Inertia
• Galileo developed the concept of Inertia
• Inertia is a property of matter that causes it to resist
changes in its velocity
• Inertia is directly proportional to mass
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Inertia
• Galileo developed the concept of Inertia
• Inertia is a property of matter that causes it to resist
changes in its velocity
• Inertia is directly proportional to mass
• As mass increases inertia increases
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Inertia
• Galileo developed the concept of Inertia
• Inertia is a property of matter that causes it to resist
changes in its velocity
• Inertia is directly proportional to mass
• As mass increases inertia increases
• As mass decreases inertia decreases
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Inertia
• Galileo developed the concept of Inertia
• Inertia is a property of matter that causes it to resist
changes in its velocity
• Mass is a quantitative measure of inertia
• As mass increases inertia increases
» Double the mass---------double the inertia
» Triple the mass -----------triple the inertia
» Half the mass ------------- half the inertia
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Inertia
• Galileo performed experiments by carefully
observing and measuring objects that rolled
down and then up inclines of various angles
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• According to Aristotle, the natural state of objects was to be
at rest, and if you got them moving, eventually they would
come to rest again.
• Galileo did experiments rolling balls down and up inclined
planes, and realized that, in the absence of some kind of
force, an object would keep moving forever once it got started
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Inertia
• It is difficult to observe objects free falling
under the influence of gravity. The objects
move fast and accelerate to quickly
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Inertia
• It is difficult to observe objects free falling
under the influence of gravity. The objects
move fast and accelerate to quickly
• By using inclines and simple trigonometry
Galileo was able analyze mass and resulting
acceleration as influenced by the constant
force of gravity.
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Newton’s 3 Laws of Motion
• 1st Law of Motion
– Often called the Law of Inertia
• Heavily influenced by Galileo’s concept of inertia
In the absence of an unbalanced applied force
(Fnet = 0), a body at rest remains at rest, and a
body already in motion remains in motion with a
constant velocity (constant speed and direction).
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Newton’s 3 Laws of Motion
• 2nd Law of Motion
– Experiments show that the acceleration of an
object is directly proportional to the force exerted
on it and inversely proportional to its mass.
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Newton’s 3 Laws of Motion
• 2nd Law of Motion
The acceleration of an object is directly proportional to
the net force acting on it and inversely proportional to
its mass. The direction of the acceleration is in the
direction of the applied net force.
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Newton’s 3 Laws of Motion
3rd Law of Motion
For every force (action), there is an equal and
opposite force (reaction).
1
1
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2nd Law of Motion
•F = ma
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2nd Law of Motion
• SI Units of Force
F = ma
1 Newton = 1 kg(1m/s2)
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2nd Law of Motion
• SI Units of Force
F = ma
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2nd Law of Motion
• SI Units of Force
F = ma
1 Newton = 1 kg(1m/s2)
2
1N = kg∙m/s
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2nd Law of Motion
• Weight
• A Force that is always directed towards the
center of the earth
• F = ma
• Weight = m(g)
• Weight is understood to be directed downward. So
it is never labeled negative
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Weight
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Weight
• What is the weight of a 12 kg mass on earth?
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Weight
• What is the weight of a 12 kg mass on earth?
Weight = m(g)
Weight = 12 kg( 9.8m/s2)
Weight = 117.6 N
= 120 N
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Weight
• Weight is perpendicular to the center of the
mass generating the gravitational field
120N
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Weight on an Incline
What is direction of weight?
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Normal Force
Normal Force is the force of a surface against an
object resting on the surface
The normal force( FN ) is always perpendicular to
the surface
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Normal Force
Normal Force is the force of a surface against an
object resting on the surface
The normal force( FN ) is always perpendicular to
the surface
The normal force will support the object on the
surface
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Normal Force
Normal Force is the force of a surface against an
object resting on the surface
The normal force equals the weight of the
object on a level surface
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Weight on an Incline
What is direction of weight?
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Normal Force on an Incline
Normal force is perpendicular to the surface
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Normal Force on an Incline
Normal force is usually less than the weight
on an incline
F N < FW
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