Physics 2211 Todays Agenda

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Physics 2211Todays Agenda

• Newtons 3rd law
• 8.1 Interacting systems
• 8.2 Action-Reaction pairs

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8.1 Interacting Systems
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Interaction

• Interaction is a mutual influence of two systems
  on each other.

The pair of forces is an action/reaction
pair. Action and reaction are simultaneous
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Long range forces
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System-Environment
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System-Environment
Forces originating in the environment are
EXTERNAL FORCES Environment doesnt move
significantly as a result of external forces
motion of the environment is of no interest.
(hammer, nail, earth)
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Newtons 2nd law
This law can be applied separately to systems 1
and 2.

• The sum includes both forces due to system 2 and
  the external forces

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Identifying Action/Reaction Pairs
? Draw each object separately. Place them in the
correct position relative to other objects. Dont
forget to include objects like the earth that may
not be mentioned in the problem. ? Identify
every force. Draw the force vector on the object
on which it acts. Label each with a subscripted
label. The usual force symbols can be
used. ? Identify the action/reaction pairs.
Force goes with force Connect the two force
vectors of each action/reaction pair with a
dotted line. When youre done, there should be no
unpaired forces. ? Identify the objects that are
systems of interest. Other objects whose motion
you dont care about are part of the
environment. ? Draw a free-body diagram for each
system of interest. Include only the forces
acting on the system, not forces that the system
exerts on other objects.
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Identifying Action/Reaction Pairs
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Systems of interest
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Propulsion

• Is a force that a system with an internal source
  of energy uses to drive itself forward.

Walk on ice foot slips and slides to walk, the
foot must stick. Static friction force on person
must act in forward direction to prevent
slipping. We walk by pushing the earth away from
us.
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Towing a car-identify all action reaction pairs
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Every force occurs as one member of an
action/reaction pair of forces

• The two members of an action/reaction pair act on
  two different objects
• The two pairs of an action/reaction pair are
  equal in magnitude but opposite in direction

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Every force occurs as one member of an
action/reaction pair of forces

• The two members of an action/reaction pair act on
  two different objects
• The two pairs of an action/reaction pair are
  equal in magnitude but opposite in direction

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Reasoning with 3rd Newtons law
Forces are equalAccelerations are not
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For a 1kg ball, this is
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The forces on accelerating boxes

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Free body diagrams
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Acceleration constraints
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Acceleration constraint is an information
independent from Newtons laws.
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• MODEL Identify which objects are systems and
  which are part of the environment. Make
  simplifying assumptions.
• VISUALIZE Pictorial representation. Show
  important points in the motion with a sketch. You
  may want to give each system a separate
  coordinate system. Define symbols and identify
  what the problem is trying to find. Include
  acceleration constraints as part of the pictorial
  model.
• Physical representation. Identify all forces
  acting on each system and all action/reaction
  pairs. Draw a separate free-body diagram for each
  system. Connect the force vectors of
  action/reaction pairs with dotted lines. Use
  subscript labels to distinguish forces, such as
  and that act independently on more than one
  system.
• SOLVE Use Newtons second and third laws
• . Write the equations of Newtons second law for
  each system, using the force information from the
  free-body diagrams.
• . Equate the magnitudes of action/reaction pairs.
• . Include the acceleration constraints, the
  friction model, and other quantitative
  information relevant to the problem.
• . Solve for the acceleration, then use kinematics
  to find velocities and positions.
• ASSESS Check that your result has the correct
  units, is reasonable, and answers the question.

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A wedge with an inclination of angle   rests
next to a wall. A block of mass   is sliding
down the plane, as shown. There is no friction
between the wedge and the block or between the
wedge and the horizontal surface.      
Find the magnitude,     , of the force that the
wall exerts on the wedge
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A girl of mass    is walking up a slippery
slope while pulling a sled of unknown mass the
slope makes an angle   with the horizontal.
The coefficient of static friction between the
girl's boots and the slope is   the
friction between the sled and the slope is
negligible. It turns out that the girl can pull
the sled up the slope with acceleration up to
  without slipping down the slope. Find the mass
of the sled    . Assume that the rope
connecting the girl and the sled is kept parallel
to the slope at all times.
(4)
(1)
(3)
(2)
(5)
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(2)