物理与光电工程学院：《大学物理》课程PPT教学课件（讲稿，英文第三版）Chapter 8 Energy and Conservation Law

788 Work and Energy Chapter 7& 8 Energy and Conservation Law 1. Work and Energy 2. Kinetic Energy (AJAE)& Work- Energy Principle 3. Conservative(保守) and nonconservative Forces 4. Potential Energy(势能) 5. Conservation of Energy

7&8 Work and Energy Chapter 7 & 8 Energy and Conservation Law 1. Work and Energy 2. Kinetic Energy （动能）& Work- Energy Principle 3. Conservative（保守） and Nonconservative Forces 4. Potential Energy（势能） 5. Conservation of Energy

788 Work and Energy New words Conservative and non- conservative forces(保守力与 非保守力) Potential energy (势能) Conservation of mechanical energy (机械能守恒) Conservation of energy(能量守恒)

7&8 Work and Energy New words Conservative and non-conservative forces（保守力与 非保守力） Potential energy（势能） Conservation of mechanical energy （机械能守恒） Conservation of energy（能量守恒）

788 Work and Energy 87-187-3 Work and Power(P147-156) Work w Provides a link between force and energy 力的空间累积效应:F,F 1. Work done by a constant force(P148) W=F·r=( F cos o)r=F△x a Fcos Work is product of the magnitude of e displacement of the force 8203T0 △x parallel to the displacement

7&8 Work and Energy §7-1&7-3 Work and Power (P147-156) Work W Provides a link between force and energy. 1. Work done by a constant force(P148) Φ Φ W F r F r F x =  = ( cos) = x  力的空间累积效应: F, r W   Work is product of the magnitude of the displacement of the force parallel to the displacement

788 Work and Energy 2 Work done by several forces ifF=h+F2+}3+F4+ W=∫FdF=「FdF+dF+,dF+ =W+W,+W2+ The work done by several forces = algebraic sum of the work done by the individual force. ◆合力的功=分力的功的代数和

7&8 Work and Energy if       F = F1 + F2 + F3 + F4 +           = + + + =  =  +  +  +     1 2 3 1 2 3 W W W W F d r F d r F dr F dr The work done by several forces = algebraic sum of the work done by the individual force. 2 Work done by several forces 合力的功 = 分力的功的代数和

788 Work and Energy 3 work done by a general variable force(35dJP152-156) 1.无限分割路径; The path is divided into short intervals. 2以直线段代替曲线段; The intervals could be treated as straight b lines 3以恒力的功代替变力的功; During each small interval. the force is approximately constant. 6 4将各段作功代数求和; The total work done is the samAr of all terms

7&8 Work and Energy 3 work done by a general variable force(变力P152-156) a b 1.无限分割路径； The path is divided into short intervals. 2.以直线段代替曲线段； The intervals could be treated as straight lines. 3.以恒力的功代替变力的功； During each small interval, the force is approximately constant. 4.将各段作功代数求和； The total work done is the sum of all terms. F2  2 F1  1 r

788 Work and Energy △W,=F△rcos AW2= F2Arcos 0, △W.=F△rcos日 +)AW=F△rcos W=∑AW=∑ F Ar cos e

7&8 Work and Energy 1 1 1 W = F r cos 2 2 2 W = F r cos  i i i W = F r cos  n n n +） W = F r cos i n i W =  W =1 i i n i F r cos 1 =  =

788 Work and Energy Letr→>0 Cost W=lim∑F;4 rcos 6 Fdrcos0=Fdr B B W=F·dF=| Fcos adr When a particle moves from A to B along a curve path, the total work done by the force equals the integral from a to B

7&8 Work and Energy Let r → 0 i i n i r W F r   lim cos 1 0 =  → = Fdr cos b a =  =  b a F dr     =  = B A B A W F dr F cosdr   Fcos A r B dr r r o When a particle moves from A to B along a curve path, the total work done by the force equals the integral from A to B

788 Work and Energy 4 Special expression in scalar product(p154) F=Fi+Fj+Fk, and dr=dxi +dyj+dck W=F·cF=(Fax+F如+F) =F+["F4+[F W=∫fdx+∫Fdy+∫Fdz 历=W+W+

7&8 Work and Energy , and ˆ ˆ ˆ F F i F j F k = x + y + z  . ˆ ˆ ˆ dr = dxi + dy j + dzk       = + + =  = + + f i f i f i z z z y y y x x x B A x y z B A F dx F dy F dz W F dr (F dx F dy F dz)   4 Special expression in scalar product(p154)    W = F x + F y + F z x d y d z d W =W x +W y +W z

788 Work and Energy 5 Work is a scalar quantity, no direction dw=F cos ]dr=Fcos ads dW= f 0°0 positive work B 90°<6<180°,dW<0 negative work drke.F 6=90°F⊥dFdW=0 zero work

7&8 Work and Energy 5 Work is a scalar quantity, no direction. 0   90 , dW  0 dW = F cos dr = F cosds  90  180 , dW  0 W F r   d = d = 90 F ⊥ dr dW = 0    F  r  d  Fi  1 dr  i r  d B * *  i  1 A F1  positive work; negative work. zero work;

788 Work and Energy 6 Geometrical Representation of Work: The work done by a force Cost equals the area under the F versus r curve 变力曲线 与位移轴在极限x,x2之间所包围ord7B 的面积 Example 7-5 and 7-6

7&8 Work and Energy 6 Geometrical Representation of Work: The work done by a force equals the area under the F versus x curve —— 变力曲线 与位移轴在极限x1, x2 之间所包围 的面积. Fcos A r B dr r r o Example 7-5 and 7-6