Physics 121, Sections 9, 10, 11, and 12 Lecture 13 Today's Topics Homework 6: due after midterm week Chapter 6: Work and Energy Definition of work Work-kinetic energy theorem Potential energy Non-conservative forces Generalized work-kinetic energy theorem Examples Physics 121: Lecture 13, Pg 1
Physics 121: Lecture 13, Pg 1 Physics 121, Sections 9, 10, 11, and 12 Lecture 13 Today’s Topics: Homework 6: due after midterm week Chapter 6: Work and Energy Definition of work Work-kinetic energy theorem Potential energy Non-conservative forces Generalized work-kinetic energy theorem Examples
Definition of work Ingredients: Force(F), displacement (Ar) Work W. of a constant force F acting through a displacement A r is F W=F△rcos0 0/△r Physics 121: Lecture 13, Pg 2
Physics 121: Lecture 13, Pg 2 Definition of Work: Ingredients: Force ( F ), displacement ( r ) Work, W, of a constant force F acting through a displacement r is: W = F r cos F r Fr
Units. Force x Distance= Work Newton x Meter Joule [M[L/2[][ML]2/[2 mks gs other N-m(Joule) Dyne-cm(erg) BTU 1054J 107J calorie = 4.184 J foot-Ib =1.356 J e∨ 1.6×10-19J Physics 121: Lecture 13, Pg 3
Physics 121: Lecture 13, Pg 3 Units: N-m (Joule) Dyne-cm (erg) = 10-7 J BTU = 1054 J calorie = 4.184 J foot-lb = 1.356 J eV = 1.6x10-19 J mks cgs other Force x Distance = Work Newton x [M][L] / [T]2 Meter = Joule [L] [M][L]2 / [T]2
Work Kinetic-Energy Theorem INet Work done on object] Change in kinetic energy of object W=△K K-K mv mV Physics 121: Lecture 13, Pg 4
Physics 121: Lecture 13, Pg 4 Work Kinetic-Energy Theorem: {Net Work done on object} = {change in kinetic energy of object} Wnet = K = K2 − K1 2 1 2 2 mv 2 1 mv 2 1 = −
Work done by Variable Force:(1D) When the force was constant we wrote W= FAX area under F vs x plot △X F(x) ∠X Physics 121: Lecture 13, Pg 5
Physics 121: Lecture 13, Pg 5 Work done by Variable Force: (1D) When the force was constant, we wrote W = Fx area under F vs x plot: F x Wg x F(x) x1 x2 x