Physics 2211: Lecture 19

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Physics 2211 Lecture 19

• MORE Newtons Third Law

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Newtons Third Law

• Forces are never lonely they always come in
  pairs.
• Each force in the (action/reaction) pair acts on
  DIFFERENT objects
• The two forces are equal in magnitude but
  opposite in direction
• (For every action, there is an equal and
  opposite reaction.)

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Reasoning with N. 3rd Law
Forces are equalAccelerations are not
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For a 1kg ball, this is
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Propulsion and N. 3rd Law
WALKING
DRIVING
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Tension Forces and N. 3rd Law
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Rope in Equilibrium (Zero Acceleration)
Action-Reaction Pair
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Action-Reaction Pair in Rope (at any point)
Action-Reaction Pair
The right end is in equilibrium, too.
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Quiz 3 Redux
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Same Result
Action-Reaction Pair
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Tension Forces Act (in effect) as
Action-Reaction Pairs
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Non-equilibrium Example
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Blocks accelerate (string, blocks are not in
equilibrium)
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Action-Reaction Pair
Action-Reaction Pair
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Thus, the string can be ignored. The tension
forces behave as an Effective action-reaction
pair
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Constraints on Motion
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Using N. 3rd Law with N. 2nd Law
? 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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A pulley/peg changes the direction of a
force without changing its magnitude
Three choices for the system
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A w 20 N block is attached to a scale. The
scale reads 20 N when the scale is attached to a
wall. What does the scale read when it is instead
attached to another w 20 N block? (1) 0 N
(2) 10 N (3) 20 N (4) 40
N
Reads 20 N
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Solution

• Draw a Free Body Diagram of one of the blocks!!

• Use Newtons 2nd Lawin the y direction

T
a 0 since the blocks are stationary
FTOT 0
w
T - w 0
T w 20 N.
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• The scale reads the tension in the rope, which is
  
• T 20 N in both cases!

T
T
T
T
T
T
T
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m
slides with friction
T

M
slides without friction

• 1 a 0 2 a 1.25 3 a
  2.45 4 a 3.0 5 a 4.9

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• A 240-lb Tech kick returner receives the opening
  kickoff. The (scrawny)
• 150-lb kicker for UGA attempts to tackle by
  colliding head-on with the Tech returner, but is
  knocked back 10 yards while the Tech player
  continues racing toward the endzone. During the
  collision, the force of the UGA kicker ON the
  Tech returner is

• Larger than (2) Equal to (3) Less than

the force of the Tech returner ON the UGA kicker.
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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)
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Recap
MORE Newtons 3rd Law
Next Lecture FRIDAY 22 OCTOBER
Rope and Pulley Problems
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Towing a car-identify all action reaction pairs
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The forces on accelerating boxes

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Free body diagrams
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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.