Chapter 4

Forces and Newton’s Laws of Motion

Topics and Activities

 

  1. Concept of force – push or pull – vector quantity!
    1. Not all forces result in motion!  Net force is what matters!
  2. Newton’s three laws:
    1. Law of Inertia

                                               i.     Inertia: tendency of an object to remain at rest

                                             ii.     Mass: measure of how much inertia object has – does not vary with location as weight does (SI UNIT: kg)

1.     mass vs. weight: balance vs. spring scale

                                            iii.     Demo: hammer, chunk of steel, block of wood

    1. ΣF = ma – a net force results in an acceleration

                                               i.     If ΣF=0 then a=0, if ΣF≠0 then a≠0.

                                             ii.     Works component-wise

                                            iii.     Unit: kgm/s2 = Newton N  (English: pound lb)

                                            iv.     EX free fall – gravity

                                             v.     **Equilibrium: net force=0, a=0 (static)

    1. Action/reaction forces – you can’t pushed without getting pushed back – force is an interaction between two things.  If A pushes B, then it follows that B is pushing back at A with the same magnitude force but opposite in direction.

                                               i.     Hewitt “Tug of War” worksheet, more practice identifying action/reaction forces

                                             ii.     Clarification: action/reaction forces act on different objects. To learn about motion of object A, consider only forces on object A.

                                            iii.     EX: demo – fingers on wall; skater pushes another, professor pushes cart, pulling sled on ice

                                            iv.     (Check questions 1-7 from notes)

  1. Examples of kinds of forces
    1. Three fundamental forces of nature: gravitational, strong nuclear, and electroweak
    2. Forces we will work with (mostly non-fundamental)

                                               i.     Weight W=mg down (gravity close to surface of Earth)

                                             ii.     Newton’s Law of Gravitation – gravity further from Earth’s surface than a fraction of an Earth radius

                                            iii.     Normal forces – due to contact with surfaces, direction perpendicular to surface

                                            iv.     Friction – static and kinetic (details later)

                                             v.     Tension force

  1. Free body diagrams: problem solving tool
    1. Rules for FBDs overview
    2. Examples
    3. Hewitt worksheets: FBD practice on rock and velocity vectors with bowling pins
    4. (Check Q’s: 8 and 9 from notes, next page of Q’s)
  2. Friction:
    1. Activity on static vs kinetic friction: exploring friction with a spring scale and PASCO friction exploration kit
    2. f=mN
    3. example problems
  3. More on forces:
    1. FBD’s and algebraic form of Newton’s 2nd Law: “Newton’s 2nd and 3rd Laws” worksheet
    2. FBD’s on an incline
    3. problems – Active Physics exercises 2.6, 2.8, 2.10, 2.12, 2.13, 2.14

 

 

Last update 1 October 2002 SLN