The forces that affect our lives, the influences that mold and shape us, are often like whispers in

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The forces that affect our lives, the influences
that mold and shape us, are often like whispers
in a distant room, teasingly indistinct,
apprehended only with difficulty.
Charles Dickens
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Energy

• Energy comes in many forms and shapes

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Energy

• Energy comes in many forms and shapes
• Food we eat is made up of complex molecules like
  carbohydrates, fats and proteins. You learned
  last year about the energy contained within
  chemical bonds and what happens when these bonds
  are rearranged

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Energy

• Energy comes in many forms and shapes
• Thermal energy is that idea of heat and what it
  means to be warm or cold and which direction
  those will flow. The idea of thermodynamics is
  the study of heat and how it behaves.
• Remember Qmc?T ??

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Energy

• Energy comes in many forms and shapes
• Electromagnetic radiation from the sun provides
  us with energy above and beyond the obvious heat
  that we feel while baking on a beach.
• The electromagnetic spectrum includes many
  varieties of waves that all possess differing
  amounts of energy.

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Energy

• Energy comes in many forms and shapes
• Einsteins biggest contribution to scientific
  thought was his astonishing revelation that all
  matterALL MATTERis simply energy packaged in
  different ways.
• Emc2 is simply a mathematical statement of that
  observation

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Forms of Energy

• Looking around you every day will lend itself to
  seeing varying forms of energy in our terms.
• Springs, elastic bands, gasoline, light, heat,
  hammers, lawnmowers, shovels, even the human
  body..these all possess or use or transform that
  idea we call energy

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Forms of Energy

• Looking around you every day will lend itself to
  seeing varying forms of energy in our terms.
• Springs, elastic bands, gasoline, light, heat,
  hammers, lawnmowers, shovels, even the human
  body..these all possess or use or transform that
  idea we call energy
• Another way to look at energy as a definition is
  where we begin Chapter 7.Energy is that entity
  that provides the ability to do work.

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Work the scientific definition

• Work as we know it in every day terms is not
  really the truest definition of work in the
  scientific realm

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Work the scientific definition

• Work as we know it in every day terms is not
  really the truest definition of work in the
  scientific realm
• In physics, and all science, work is done when a
  force causes a movement of an object.

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Work the scientific definition

• Work as we know it in every day terms is not
  really the truest definition of work in the
  scientific realm
• In physics, and all science, work is done when a
  force causes a movement of an object.
• IMPULSE force applied over a period of time
  (F)(t)
• WORK force applied over a distance (F)(x)

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WORK

• When a force acts upon an object to cause a
  displacement of the object, it is said that work
  was done upon the object.
• There are three key ingredients to work - force,
  displacement, and cause.

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WORK

• When a force acts upon an object to cause a
  displacement of the object, it is said that work
  was done upon the object.
• There are three key ingredients to work - force,
  displacement, and cause.
• In order for a force to qualify as having done
  work on an object, there must be a displacement
  and the force must cause the displacement.
• There are several good examples of work which can
  be observed in everyday life - a horse pulling a
  plow through the field, a father pushing a
  grocery cart down the aisle of a grocery store, a
  freshman lifting a backpack full of books upon
  her shoulder, a weightlifter lifting a barbell
  above his head, an Olympian launching the
  shot-put, etc. In each case described here there
  is a force exerted upon an object to cause that
  object to be displaced.

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Read the following four statements and determine
whether or not they represent examples of work.

• A teacher applies a force to a wall and becomes
  exhausted. 
•  
• A book falls off a table and free falls to the
  ground. 
•  
• A waiter carries a tray full of meals above his
  head by one arm straight across the room at
  constant speed. (Careful! This is a very
  difficult question which will be discussed in
  more detail later.) 
•  
• A rocket accelerates through space. 
•  
•  

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WORK

• Mathematically, work can be expressed by the
  following equation
• W (F)(x)(cos?)
• where F is the force, x is the displacement,
  and the angle (?) is defined as the angle between
  the force and the displacement vector.

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Units are the punctuation of science!!!

• Unit for work is a derived one
• Force times displacement
• Newtons times meters (Nm)
• Nm is called the JOULE
• 1000 Joules is 1 kilojoule (1 kJ)
• 1,000,000 Joules is 1 megajoule (1 MJ)

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WORK

• The angle measure is defined as the angle between
  the force and the displacement. To gather an idea
  of its meaning, consider the following three
  scenarios

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WORK Scenarios

• Scenario A A force acts rightward upon an object
  as it is displaced rightward. In such an
  instance, the force vector and the displacement
  vector are in the same direction. Thus, the angle
  between F and x is 0 degrees.

Force
T 0
displacement
Example Pulling on a crate with a rope
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WORK Scenarios

• Scenario B A force acts leftward upon an object
  which is displaced rightward. In such an
  instance, the force vector and the displacement
  vector are in the opposite direction. Thus, the
  angle between F and x is 180 degrees.

Force
T 180
displacement
Example friction acting against the crate from
Scenario A
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WORK Scenarios

• Scenario C A force acts upward on an object as
  it is displaced rightward. In such an instance,
  the force vector and the displacement vector are
  at right angles to each other. Thus, the angle
  between F and x is 90 degrees.

Force
displacement
T 90
Example a waiter carrying a tray of food across
a dining room
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Work Formula

• Where does that cosine ? come from?
• Consider an example of pulling on a crate with a
  rope
• A. If the pull and the displacement are in the
  same direction, then ? is 0.
• COSINE OF ZERO 1.0

Fpull
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Work Formula

• Where does that cosine ? come from?
• Consider an example of pulling on a crate with a
  rope
• B. If the pull and the displacement are in
  opposite directions, then ? is 180.
• COSINE OF 180 -1.0

Ffriction
displacement
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Work Formula

• Where does that cosine ? come from?
• Consider an example of pulling on a crate with a
  rope
• C. If the pull and the displacement are at an
  angle of 37 then
• The portion of the force that is
• parallel to the motion is (F)(cos?)
• Assuming the displacement is in the same
  direction as that parallel force, then the
  formula looks like
• W(F)(cos?)(x)

Fpull
?
F cos?
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In-Class PracticeSolve the following examples
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Solutions

• Diagram A Answer
• W (100 N) (5 m) cos(0 degrees) 500 J
• The force and the displacement are given in the
  problem statement. It is said (or shown or
  implied) that the force and the displacement are
  both rightward. Since F and x are in the same
  direction, the angle is 0 degrees.

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Solutions

• Diagram B Answer
• W (100 N) (5 m) cos(30 degrees) 433 J
• The force and the displacement are given in
  the problem statement. It is said that the
  displacement is rightward. It is shown that the
  force is 30 degrees above the horizontal. Thus,
  the angle between F and x is 30 degrees.

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Solutions

• Diagram C Answer
• W (150 N) (5 m) cos(0 degrees) 750 J
• The displacement is given in the problem
  statement. The applied force must be 150 N since
  the 15-kg mass (Fgrav150 Nweight) is lifted at
  constant speed. Since F and y are in the same
  direction, the angle is 0 degrees.

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Practice Problems

• A 10-N force is applied to push a block across a
  frictionless surface for a displacement of 5.0 m
  to the right.
• Which of these forces does work on the box?
• Calculate the work done.

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Practice Problems

• A 10-N force is applied to push a block across a
  frictionless surface for a displacement of 5.0 m
  to the right.
• Only Fapp does work. Fgrav and Fnorm do not do
  work since a vertical force cannot cause a
  horizontal displacement.
• WFcos?x(10N)(cos0)(5m)50 J

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Practice Problems

• A 10-N frictional force slows a moving block to a
  stop after a displacement of 5.0 m to the right.
• Which of these forces does work on the box?
• Calculate the work done.

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Practice Problems

• A 10-N frictional force slows a moving block to a
  stop after a displacement of 5.0 m to the right.
• Only Ffrict does work. Fgrav and Fnorm do not do
  work since a vertical force cannot cause a
  horizontal displacement.
• W Fcos?x (10N)(cos180)(5m) -50 J

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Practice Problems

• A 10-N force is applied to push a block across a
  frictional surface at constant speed for a
  displacement of 5.0 m to the right.
• Which of these forces does work on the box?
• Calculate the work done.

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Practice Problems

• A 10-N force is applied to push a block across a
  frictional surface at constant speed for a
  displacement of 5.0 m to the right.
• Fapp and Ffrict do work. Fgrav and Fnorm do not
  do work since a vertical force cannot cause a
  horizontal displacement.
• WappFappcos?x (10N)(cos0)(5m) 50 J
• WfrictFfrictcos?x (10N)(cos180)(5m) -50 J

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Practice Problems

• An approximately 2-kg object is sliding at
  constant speed across a friction free surface for
  a displacement of 5 m to the right.
• Which of these forces does work on the box?
• Calculate the work done.

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Practice Problems

• An approximately 2-kg object is sliding at
  constant speed across a friction free surface for
  a displacement of 5 m to the right.
• Neither of these forces do work. Forces do not do
  work when they make a 90-degree angle with the
  displacement.
• WFcos?x (20N)(cos90)(5m) 0 J

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Practice Problems

• An approximately 2-kg object is pulled upward at
  constant speed by a 20-N force for a vertical
  displacement of 5 m.
• Which of these forces does work on the object?
• Calculate the work done.

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Practice Problems

• An approximately 2-kg object is pulled upward at
  constant speed by a 20-N force for a vertical
  displacement of 5 m.
• Both Fgrav and Ftens do work. Forces do work when
  there is some component of force in the same or
  opposite direction of the displacement.
• Wgrav Fgravcos?x (20N)(cos180)(5m) -100 J
• Wtens Ftenscos?x (20N)(cos0)(5m) 100 J