# Teaching Introductory Physics by Arons Chapter 3 Erin

• Slides: 14

Teaching Introductory Physics by Arons Chapter 3 Erin Stefanik & Carl Wenning

Overview of Chapter 3 • • • Overcoming Student Preconceptions Newton’s Laws of Motion Terminology and Preconceptions Understanding Gravity Definitions versus Equalities Problem Solving Approaches

Student Preconceptions • Area of greatest concentration - dynamics • Preconceptions are rooted in everyday experiences and hard to overcome • Mastery of subject matter tied up with conceptual understanding - insufficient: – Lectures, demonstrations, calculations • Required are: – Identification, confrontation and resolution along with qualitative thinking and testing

Some Examples from FCI • Try these questions from the Force Concept Inventory - nearly every question is based on one form of preconception or another…

Laws of Motion • Laws of Motion – as they apply to me • 1 st Law: I’m lazy: I won’t move if someone doesn’t push me, and likewise I won’t stop moving if someone won’t push me • 2 nd Law: I can’t do anything alone: If you push me, I’ll either move faster or slower (depending on how you push me) • 3 rd Law: I believe in “Eye for an eye”: If you push me, I’ll push back

Terminology & Preconceptions • “force” and “mass” – what do they mean to you? • Two approaches to a description – Ernst Mach’s method – describe inertial mass first, the force as a result of an accelerating inertial mass – see Weinstock(1961) • Benefit: greater “internal consistency” – also, helps establish an understanding of inertial mass as something different than gravitational mass – Relativity • Problem: often described in texts quite cryptically and is difficult to grasp – Newton’s method – describe forces first • Most common method – probably how you learned • Does it matter which? Most important is the “internal consistency of the network one elects to form” • Take-home message: Have a plan for approaching this and be aware of preconceived notions of “force” and “mass” -- provide operational definitions.

Tips for Teaching N 1 • Common examples: – Table cloth yanked out from under dishes – Massive block remains at rest despite string pulled from beneath • First works because coefficient of friction is low • Second works because string stretches until it breaks

Tips for Teaching N 2 • Distinguish “weight” from “mass” of an object. • Use “accelerate” instead of “move” • Force is not a material property – emphasize the interaction • Be cautious with superposition – often students lose sight of the individual forces’ effects • Explicitly describe difference in a “shove” and a continuous force • Forces “act” on bodies, not “do work” on bodies – helps avoid confusion in later concept of work

Tips for Teaching N 3 • Preconception: Forces cannot be exerted by “rigid” or inert objects. • Distinguish between active and passive • Cleary state N 3: avoid “for every action there is a reaction” • Deal effectively with non-contact forces (don’t leave Earth out of consideration) • Carefully draw free-body diagrams

The Gravity of Gravity • Why do things fall? Because of gravity! – Common cause and effect statement that only implies understanding • Meaning of “vertical” and “horizontal” • Air does not produce gravity - while air “presses down on things, it is not the source of gravity. ” • g is not “gravity” • Weight, weightlessness, and free fall – Gravity is not limited to the surface of the Earth

Fictitious and Frictional Forces • When one accelerates, one is not: – Thrown backward (rectilinear motion) – Pushed sideways (circular motion) – Free of the pull of gravity (freefall) • Friction is a passive, reactive force – Friction does not always oppose motion – Friction often assists motion such as in walking

Definitions versus Equalities • Be careful to state the differences present here. • Most books do not yet use (≡) in the 2 nd equation

Problem Solving Approaches • Occasionally, include irrelevant details in the problem • Require a complete algebraic solution prior to plugging in numbers • Numerical answers do not imply understanding • Set up free-body diagrams with a coordinate system, then apply the law.