Forces: Maintaining Equilibrium or Changing Motion

1
Chapter 4

• Forces Maintaining Equilibrium or Changing
  Motion

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Force Defined

• Push or Pull
• Ability to accelerate
• Ability to start, stop, speed up, slow down or
  change direction of an object
• Measured in the Newton (N) the force required
  to accelerated 1 kg mass 1m/s/s

3
Force is a Vector

• Forces have both Magnitude and Direction.
• Force Characteristics
• Point of application
• Line of action
• Magnitude
• Direction

4
Internal Forces

• Internal forces act within the object or system
  whose motion is being examined.
• Tensile Forces Pulling forces
• Compressive Forces Pushing forces
• What is structural Failure?
• Internal forces are incapable of producing
  changes in motion of a systems COG.

5
External Forces

• Forces that act on an object as a result of
  interaction with the environment.
• Normal Force (force Perpendicular to the
  contacting surface)
• Frictional Force (force Parallel to the surfaces
  in contact and opposes motion or sliding between
  the surfaces)

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Dry Friction

• Coulomb Friction
• Friction arises as result of interactions between
  molecules of surfaces in contact.
• Static/Dynamic Friction

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Friction and Normal Force

• Increasing weight increases normal contact force
  acting on the objects which increases the
  interactions of molecules of the contacting
  surfaces because they are pushed together harder.
• With increase in normal force friction force
  increases proportionally.

8
Friction and Surface Area

• Increases in Surface area contact between two
  objects does not increase friction.
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Static Friction vs Dynamic Friction

• Static Friction is greater or less than Dynamic
  friction?
• How does the material making up the objects
  moving past each other affect friction?

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Force Composition

• Net Forcesum of all forces acting on an object
  or system. The vector sum of all the external
  forces (magnitude and direction).
• Resultant force-the results of vector addition of
  forces.

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Colinear Forces

• Forces that have the same line of action. They
  may be in the same direction or acting in
  opposite directions.
• Tug of war
• Running into the wind
• Pushing a fridge across the floor

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Lets do the math!

• Pushing on a fridge with 700N of Force
• Resistive force of fridge is -900N.
• Resultant force is -200N. The fridge wont move

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More Math!

• Tug of War from book
• 3 people each pulling with 100N, 200N and 400N
  of force on a rope. 700N of force on rope.
• Other team-200N, -200N, -200N of force on the
  rope. -600N of force on rope.
• The resultant force on rope is 100 N of force

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Concurrent Forces

• Forces that do not act along the same line, but
  they do act through the same point.
• Gymnast Example