Physics 2211, Spring 2005

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Physics 2211 Lecture 17

• Newtons Third Law

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Newtons Third Law

• For every action there is an equal and opposite
  reaction.
• Each force in the (action/reaction) pair acts on
  DIFFERENT objects.
• Example, gravity

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Newton's Third Law
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Example of Fuzzy Thinking
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Example of Good Thinking

• Consider only forces on the box!

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ExampleNewtons 3rd Law

• Two blocks are stacked on the ground. How many
  action-reaction pairs of forces are present in
  this system?

(1) 2 (2) 3 (3) 4
a
b
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ExampleNewtons 3rd Law
(3) 4
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Newtons 3rd Law

• Consider the following case

What are the all the forces acting on the P
plank F floor W wall E earth ?
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Newtons 3rd Law

• To determined the dynamics of the plank, consider
  only the forces acting on the plank.

• Isolate the plank from
• the rest of the world
• (free body diagram).

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Newtons 3rd Law

• What are the forces acting on the plank ?

• Isolate the plank from
• the rest of the world
• (free body diagram).

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Gravity and Newtons 3rd Law
Forces are equalAccelerations are not
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Gravity and Newtons 3rd Law
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Propulsion and Newtons 3rd Law
WALKING
DRIVING
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The forces on accelerating boxes

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Free body diagrams
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Tension Forces and Newtons 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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Replace the wall with another person
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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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Towing a car-identify all action reaction pairs
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Using Newtons 3rd Law with Newtons 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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m
slides with friction
T

M
slides without friction

• 1 a 0 m/s2 2 a 1.25 m/s2 3 a 2.45
  m/s2 4 a 3.0 m/s2 5 a 4.9 m/s2

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m

M
slides without friction
Cannot choose m M as the system. Why?
Choose m as the system
Now choose M as the system. Choose to the left
as positive. From Newtons 2nd and 3rd Laws
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Work this problem before recitation on Monday
A girl of mass mG is walking up a slippery slope
while pulling a sled of unknown mass the slope
makes an angle q with the horizontal. The
coefficient of static friction between the girl's
boots and the slope is mS 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 a without slipping down
the slope. Find the mass of the sled mS. Assume
that the rope connecting the girl and the sled is
kept parallel to the slope at all times.
(4)
(2)
(1)
(5)
(3)