Forces%20and%20the%20Laws%20of%20Motion%20Changes%20in%20Motion

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Forces and the Laws of MotionChanges in Motion

• Chapter 4Section 1

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Learning Targets

• Describe how force affects the motion of an
  object
• Interpret and construct free body diagrams

P3.1A, P3.2A
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What is Force???

• Forces describe the interactions between objects
  and their environment
• You exert a force on a ball when you throw it or
  kick it
• You exert a force on a chair when you sit in it

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Units of Force

• The SI unit of force is the newton
• The newton (N) is defined as the amount of force
  that, when acting on a 1 kg mass, produces and
  acceleration of 1 m/s2
• Therefore, 1 N 1 kg x 1m/s2

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Contact Forces

• Forces can act through contact or at a distance
• Contact forces result from physical contact
  between two objects
• Stretching a spring, pulling a wagon, catching a
  football
• Usually easy to identify

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Field Forces

• Field forces do not involve physical contact
  between two objects
• Gravity, attraction or repulsion between electric
  charges
• According to the theory of fields, masses create
  gravitational fields in the space around them

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Reviewing Forces

• Remember that a force is a push or pull on an
  object that results from the interactions between
  objects and their environment
• Force is a vector quantity because it has both a
  magnitude and a direction

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Balanced vs. Unbalanced Forces

• An object that is experiencing a balanced force,
  will remain at rest
• Since balanced forces are of equal magnitude and
  in opposite directions, they balance each other
• Objects experiencing unbalanced forces will
  undergo a change in motion

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Forces and Acceleration

• Forces can cause accelerations
• In many situations, a force exerted on an object
  can change the objects velocity
• A change in velocity means an object is
  accelerating
• Forces can cause stationary objects to move,
  moving objects to stop, or moving objects to
  change direction

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Relating Force and Weight

• The weight of an object is a measure of the
  magnitude of the gravitational force exerted on
  the object
• Many of the terms and units used every day to
  talk about weight are really units of force that
  can be converted to newtons

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Force Diagrams

• Force has both a magnitude and a direction
• Therefore it is a vector
• Diagrams that show force vectors as arrows are
  called force diagrams

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Drawing Force Diagrams

• When drawing a force diagram ,the tail of the
  vector arrow is attached to the object on which
  the force is acting
• A force vector points in the direction of the
  force, and its length is proportional to the
  magnitude of the force

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Free-Body Diagrams

• While force diagrams represent all the forces
  acting in a situation, free-body diagrams
  represent only the forces acting on the object of
  interest
• The force exerted by the object on other objects
  are not included in the free-body diagram because
  they do not affect the motion of the object

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Force Diagram Free-Body Diagram
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Drawing Free-Body Diagrams

• A physics book is at rest on a desk.
  Gravitational force pulls the book down. The
  desk exerts an upward force on the book that is
  equal in magnitude to the gravitational force.
  Draw a free-body diagram of the book.
• Draw a free-body diagram of a person pulling a
  sled. The magnitudes of the forces acting on the
  sled are 60 N by the string, 130 N by the Earth
  (gravitational force), and 90 N upward by the
  ground