I believe that a scientist looking at nonscientific problems is just as dumb as the next guy. -Richard Feynman Nobel Prize in Physics, 1965

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I believe that a scientist looking at
nonscientific problems is just as dumb as the
next guy. -Richard FeynmanNobel Prize in
Physics, 1965
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Skill Goal for Chapter 4

• Draw a correct free body diagram (FBD) for every
  object in a described scenario

• 0. FBDs are only done in inertial
  (non-accelerating) frames of reference
• Use to figure out if any
  forces are missing, and what forces (or
  components) have to balance
• Include on forces ON a body, not forces exerted
  BY a body
• Use 3rd law (equal and opposite reaction force
  to help get forces on different bodies
• Note Acceleration is not a force.

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For which scenario is FN larger?Choose A, B or
C FN is the same for both.

• For which scenario is Ff larger?
• Choose A, B, or
• C Ff is the same for both.

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2. Include only forces ON the body of interest

• When I push on a wall and stand at a steep angle,
  what force shown should not be included in a FBD
  of me?
• My weight, mg
• The normal force from the floor
• My push on the wall
• Friction with the floor

• What can you conclude about the push of the wall
  on me (Fw) and the force of friction from the
  floor (Ff)?
• Ff gt Fw D. Either A or B
• Ff Fw E. Either B or C
• Ff lt Fw

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3rd Law Equal and Opposite

• As I stand still, not accelerating, what is the
  equal and opposite reaction force to the force
  of gravity on me (that is, my weight)?
• The floor pushing up on me
• My gravitational pull on the Earth
• Neither of these.