Why the Logical Order of Physics is Illogical

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Why the Logical Order of Physics is Illogical

• David Montalvo
• dmontalvo_at_upper-merion.k12.pa.us

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Caveat Lector

• I cite no literature
• I do not pretend that this idea is original
• Take this in the spirit of a simple observation

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The way an introductory physics course normally
begins...

• We start with motion
• We then move to mass and forces
• From there we go to energy-momentum
• This is a logical order

to us
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What makes this order logical?

• It builds from the simplest dimensions. In MKS
  meters and seconds to kilograms to Newtons to
  kgm/s and Joules

We think of motion as most basic because we
derive all that follows from dimensionally
simple bits
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In short, we begin by going from the what of
motion to the why of forces
So whats wrong with that?
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Motion is conceptually very diverse

• Position
• Time
• Change in Position
• Distance traveled
• Speed
• Velocity
• Change in velocity
• Acceleration

Not to mention three or four equations relating
them for a constant acceleration
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Instead why not start with conceptually more
unified material?

• An intuitive notion of force
• Interactions action and reaction
• The idea of net force
• Relating net force to motion
• Keep motion vague
• Finally introduce Newtons Second Law of Motion

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Motion comes next

• As a refinement of the notion of acceleration
• Then the Four Basic Forces came as a refinement
  of the concept of force
• In fact, I unwittingly began making a concept map
  as I kept track of the equations on the board

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Dymanics-Kinematics-Forces Concept Map
a
a
F
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What came out of this...

• It became apparent to my classes that
  acceleration is the key to relating Dynamics to
  Kinematics
• Newtons 2nd Law became the central concept of
  the first marking period

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What came out of this...

• I found myself often introducing intuitive
  concepts which were then refined later, rather
  than introducing overarching laws, and then
  applying them to specific cases
• With the Four Basic Forces came the notion of
  field and central forces

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What came out of this...

• Electrostatics was integrated into Mechanics
• From planetary gravity, it was a smooth
  transition to near-Earth uniform gravitational
  fields and projectile motion.

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Next Came Systems and Conservation

• Energy
• Center of Mass
• Momentum
• Circuits

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Systems - Conservation
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What came out of this

• Conservation became the central concept
• Electric Potential Energy was integrated into
  Energy
• Circuits fit in naturally after discussions of
  Energy and Momentum (collisions!)
• Note that we have Conservation of Current in
  Series and Voltage in Parallel

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To round out the year...

• Waves and Vibrations
• The key concept being Simple Harmonic Motion

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• This Presentation can be viewed on my website