Why the Logical Order of Physics is Illogical

1. Why the Logical Order of Physics is Illogical David Montalvo dmontalvo@upper-merion.k12.pa.us

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

3. 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

4. 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

5. In short, we begin by going from the “what” of motion to the “why” of forces So what’s wrong with that?

6. 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

7. 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 Newton’s Second Law of Motion

8. 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

9. Dymanics-Kinematics-Forces Concept Map a a F

10. What came out of this... • It became apparent to my classes that acceleration is the key to relating Dynamics to Kinematics • Newton’s 2nd Law became the central concept of the first marking period

11. 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

12. 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.

13. Next Came Systems and Conservation • Energy • Center of Mass • Momentum • Circuits

14. Systems - Conservation

15. 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

16. To round out the year... • Waves and Vibrations • The key concept being Simple Harmonic Motion

17. www.mrmont.com/teachers This Presentation can be viewed on my website: