Ch.%204%20Forces

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Ch. 4 Forces
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• Describe an objects acceleration in terms of its
  mass and the net force
• acting on it.
• Predict the direction and magnitude of the
  acceleration caused by a known
• net force.
• 3. Identify action-reaction pairs.

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Newtons 2nd Law

• Force is proportional to mass and acceleration.
• The acceleration of an object is directly
  proportional to the net external force acting on
  the object and inversely proportional to the
  objects mass.
• The direction of the acceleration is in the same
  direction as the applied force

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• Newtons 2nd Law of Motion (N2), the Law of
  Acceleration
• The acceleration of an object is directly
  proportional to the net force acting on the
  object
• a SF
• m

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• .
• Comment The essence of Newtons 2nd Law is that
  an object WILL ACCELERATE if and only if the
  EXTERNAL FORCES acting on it are NOT BALANCED. A
  Free Body Diagram is used to depict the external
  forces and Newtons 2nd Law ties the dynamics to
  the kinematics. Each external force includes a
  type, a direction, and a source.

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Newtons 2nd Law

• F ma
• Force (F) has units of Newtons (N)
• Mass (m) has units of kilograms (kg)
• Acceleration (a) has units of meters per second
  squared (m/s2)

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In other words..using the same amount of force.
Small acceleration
a
Large Mass
Large acceleration
a
Small Mass
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• Roberto and Laura are studying across from each
  other at a wide table. Laura slides a 2.2 kg
  book towards Roberto. If the net external force
  acting on the book is 2.6 N to the right, what is
  the books acceleration?

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Newtons 3rd Law

• For every action there is an equal and opposite
  reaction
• Force always exist in pairs, even field forces
• Pairs are known as action-reaction pairs

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• Newtons 3rd Law of Motion (N3), the Law of
  Interaction
• When one object exerts a force on a second object
  the second object exerts the same magnitude force
  in the opposite direction on the first object
  SIMULTANEOUSLY.
• Comment The essence of Newtons 3rd Law is
  that all forces are actually pairs in which the
  objects force each other simultaneously and
  oppositely REGARDLESS of the MOTION of EITHER.
  However, since we NEVER SEE FORCES we cannot see
  this relationship.

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Action and Reaction pairs

• Action-reaction pairs do not imply that the net
  force on either object is zero.
• The action-reaction forces are equal and opposite
  but either object may still have a net force on
  it.

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Earth / Apple (cont.)
The products are the same, since the forces are
the same.
a m
m
a
Apples little mass
Earths big mass
Earths little acceleration
Apples big acceleration
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• Identify the action-reaction pair in the
  following situations.
• A person takes a step
• A snowball hits someone in the back

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• Identify the action-reaction pair in the
  following situations.
• A baseball player catches a ball
• A gust of wind strikes a window

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• The forces acting on a sailboat are 390 N north
  and 180 N east. If the boat has a mass of 270
  kg, what are the magnitude and direction of its
  acceleration?

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• If a locomotive can exert a constant pull of
  7.5x105 N, how long would it take to increase the
  speed of the train from rest to 85 km/h?
  (disregard friction)