康涅狄格州大学：《普通物理》课程PPT教学课件（英文版）Lecture 12 Review session Friday

Physics 121, Sections 9, 10, 11, and 12 Lecture 12 Today's Topics Homework 5: Due Friday Oct 7@6: 00PM Ch.5:#9,11,12,23,26,30,32,41,57,and63. Review session Friday Oct 7@ 11: 00AM in IMS-20 Chapter 6: Work and Energy Forms of energy Definition of work Work-kinetic energy theorem Examples Physics 121: Lecture 12, Pg 1

Physics 121: Lecture 12, Pg 1 Physics 121, Sections 9, 10, 11, and 12 Lecture 12 Today’s Topics: Homework 5: Due Friday Oct. 7 @ 6:00PM Ch.5: # 9, 11, 12, 23, 26, 30, 32, 41, 57, and 63. Review session: Friday Oct. 7 @ 11:00AM in IMS-20 Chapter 6: Work and Energy Forms of energy Definition of work Work-kinetic energy theorem Examples

Summary (with comparison to 1-D kinematics) Angular Linear a= constant a= constant V= Vo+at 0=00+Oot+ ot+at And for a point at a distance R from the rotation axis X=RO V=OR a= ar Physics 121: Lecture 12, Pg 2

Physics 121: Lecture 12, Pg 2 Summary (with comparison to 1-D kinematics) Angular Linear  = constant  = 0 +0 +  1 2 2 t t a = constant v = v + at 0 x = x + v t + at 0 0 1 2 2 And for a point at a distance R from the rotation axis: x = R v = R a = R

Work Energy One of the most important concepts in physics Alternative approach to mechanics Many applications beyond mechanics Thermodynamics(movement of heat Quantum mechanics ery useful tools You will learn new(sometimes much easier) ways to solve problems Physics 121: Lecture 12, Pg 3

Physics 121: Lecture 12, Pg 3 Work & Energy One of the most important concepts in physics. Alternative approach to mechanics. Many applications beyond mechanics. Thermodynamics (movement of heat). Quantum mechanics... Very useful tools. You will learn new (sometimes much easier) ways to solve problems

Forms of Energy Kinetic: Energy of motion A car on the highway has kinetic energy We have to remove this energy to stop it The breaks of a car get Hot! This is an example of turning one form of energy into another(thermal energy) Kinetic energy is given by: K=(1/2)mv2 Physics 121: Lecture 12, Pg 4

Physics 121: Lecture 12, Pg 4 Forms of Energy Kinetic: Energy of motion. A car on the highway has kinetic energy. We have to remove this energy to stop it. The breaks of a car get HOT ! This is an example of turning one form of energy into another (thermal energy). Kinetic energy is given by: K = (1/2) mv2

Energy Conservation Energy cannot be destroyed or created Just changed from one form to another We say energy is conserved! True for any isolated system i. e when we put on the brakes, the kinetic energy of the car is turned into heat using friction in the brakes. The total energy of the " car-breaks-road-atmosphere" system is the same The energy of the car alone" is not conserved > It is reduced by the braking Doing "work on an otherwise isolated system will change it's energy Physics 121: Lecture 12, Pg 5

Physics 121: Lecture 12, Pg 5 Energy Conservation Energy cannot be destroyed or created. Just changed from one form to another. We say energy is conserved ! True for any isolated system. i.e when we put on the brakes, the kinetic energy of the car is turned into heat using friction in the brakes. The total energy of the “car-breaks-road-atmosphere” system is the same. The energy of the car “alone” is not conserved... »It is reduced by the braking. Doing “work” on an otherwise isolated system will change it’s “energy

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 12, Pg 6

Physics 121: Lecture 12, Pg 6 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

Definition of work Only the component of Falong the displacement is doing work EXample: Train on a track Fcos 0 Physics 121: Lecture 12, Pg 7

Physics 121: Lecture 12, Pg 7 Definition of Work... Only the component of F along the displacement is doing work. Example: Train on a track.  r F  F cos 

Lecture 12 ACT 1 Work a box is pulled up a rough(u>0)incline by a rope-pulley weight arrangement as shown below How many forces are doing work on the box (a)2 (b)3 (C) Physics 121: Lecture 12, Pg 8

Physics 121: Lecture 12, Pg 8 Lecture 12, ACT 1 Work A box is pulled up a rough (m > 0) incline by a rope-pulley￾weight arrangement as shown below. How many forces are doing work on the box ? (a) 2 (b) 3 (c) 4

Work: 1-D EXample (constant force) A force F= 10N pushes a box across a frictionless floor for a distance ax= 5m F △X Work done by on box F=F.△X=F△x (since F is parallel to AX) WF=(10N)x(5m)=50N-m Physics 121: Lecture 12, Pg 9

Physics 121: Lecture 12, Pg 9 Work: 1-D Example (constant force) A force F = 10N pushes a box across a frictionless floor for a distance x = 5m. x F Work done by F on box : WF = F ·x = F x (since F is parallel to x) WF = (10 N)x(5m) = 50 N-m

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 12, Pg 10

Physics 121: Lecture 12, Pg 10 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