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

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 1.Use to figure out if any forces are missing, and what forces (or components) have to balance 2.Include on forces ON a body, not forces exerted BY a body 3.Use 3rd law (“equal and opposite reaction force” to help get forces on different bodies 4.Note: Acceleration is not a force.

For which scenario is F N larger? Choose A, B or C: F N is the same for both. For which scenario is F f larger? Choose A, B, or C: F f is the same for both.

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? A.My weight, mg B.The normal force from the floor C.My push on the wall D.Friction with the floor What can you conclude about the push of the wall on me (F w ) and the force of friction from the floor (F f )? A.F f > F w D. Either A or B B.F f = F w E. Either B or C C.F f < F w

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)? A.The floor pushing up on me B.My gravitational pull on the Earth C.Neither of these.