1D Dynamics The Normal Force Consider the textbook on the table F FG mg F m mg Consider Newton s 2nd law in y direction F y F mg ma y but book is at rest So ay 0 gives F y F mg 0 F mg New force has same magnitude as the weight but opposite direction New force is a result of the contact between the book and the table New force is called the Normal Force n N or FN In general it is not equal to mg we must usually solve for N Normal means perpendicular to the surface of contact Now apply an additional force FA to the book FA N FA N F N mg FA 0 N mg FA y mg mg The normal force is not mg Normal Force Revisited Demos 4 and 7 Put textbook on a scale in an elevator N N mg a mg If elevator is at rest or moving with a constant velocity up or down a 0 Then Newton s 2nd law gives F y N mg 0 or N mg W If elevator is accelerating F N mg ma or N mg ma N m g a y If a 0 N mg If a 0 N mg If a g N 0 weightless Book on an Incline Frictionless y FBD N mg x mg y N x Using Newton s 2nd Law find the normal force and the acceleration of the book As we did for 2D kinematics break problem into x and y components F x ma x F y ma y mgsin max ax gsin N mgcos may 0 N mgcos If 0 ax 0 and N mg If 90 ax g N 0
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