weight, normal force, tension, friction notes

WEIGHT (W or FG)

  • Force due to gravity
  • At-distance-force (no contact necessary)
  • Pulling (toward) force (not pushing)
  • Direction - toward the center of Earth (DOWN, 270°)
  • Depends on mass (directly) but it’s not the same
  • Depends on g (directly)
  • Depends on location (mass does not) because g depends on location
  • W = mg
  • Being weightless can be different than feeling weightless. The normal force determines how heavy you feel. 

     

    NORMAL FORCE (FN)

  • Contact force (contact is necessary)**
  • Pushing (away) force (not pulling)
  • Direction – perpendicular to the surface (which can be vertical, horizontal or any other angle)
  • Can vary or adjust to a limit
  • Determines how heavy you feel

    ** Is possible to equal zero even with contact 

    TENSION (FT)

  • Exists in ropes, strings, chains, etc…**
  • Pulling (toward) force (not pushing)
  • Direction – along the string (which can be vertical, horizontal or any other angle)
  • Can vary or adjust to a limit

** Is possible to equal zero even when taut

FRICTION (FFR)

  • Force between surfaces due to roughness
  • Contact force (contact is necessary)**
  • Pushing (away) or pulling (toward) force
  • Direction – parallel to the surface (which can be vertical, horizontal or any other angle)
  • Direction – usually (not always) opposes the motion or impending motion
  • Depends on the normal force (directly)
  • Depends on the surface – coefficient of friction (µ) (directly) which is a unit-less quantity
  • Can be static (stationary) or kinetic (moving)
  • Static friction can vary or adjust to a limit
  • FFR <= µSFN (static)
  • FFR = µKFN (kinetic)

** Is possible to equal zero even with contact