复旦大学：《大学物理》课程教学资源（PPT课件，英文）Ch.12 Energy II：Potential energy

Ch. 12 Energy il Potential energy Ch 12 Energy II: Potential energy

Ch.12 Energy II: Potential energy Ch. 12 Energy II: Potential energy

12-1 Conservative forces(保守力) Kinetic energy <> velocity Potential energy? It is defined only for a certain class of forces called conservative forces What are conservative forces Do spring force, gravitational force, and frictional force et al. belong to conservative forces? Ch 12 Energy II: Potential energy

Ch.12 Energy II: Potential energy 12-1 Conservative forces(保守力) Potential energy? It is defined only for a certain class of forces called conservative forces. Do spring force, gravitational force, and frictional force et al. belong to conservative forces? Kinetic energy Velocity What are conservative forces?

1.The spring force Fig 12-1 (a) Fig11-13 0000 Relaxed length )100000000 00 d 00000 (e) k( The total work done by the spin force is zero in the process from(a) to(e) (round trip Ch 12 Energy II: Potential energy

Ch.12 Energy II: Potential energy 1. The spring force Fig 12-1 x 0 0 d d -d (a) (b) (c) (d) (e) ( ) 2 1 2 2 s f i W = − k x − x o x Relaxed length Fig 11-13 The total work done by the spin force is zero in the process from (a) to (e) (round trip). 0 0

2. The force of gravity See动画库/力学夹/2-04功的计 算举例(2) The total work done by the gravity is zero during the round trip If the gravitational force is not constant. is there still such behavior of the work Ch 12 Energy II: Potential energy

Ch.12 Energy II: Potential energy The total work done by the gravity is zero during the round trip. 2. The force of gravity If the gravitational force is not constant, is there still such behavior of the work? See 动画库/力学夹/2-04功的计 算举例 (2)

3. The frictional force See动画库/力学夹/204功的计 算举例(1) The total work done by frictional force is not zero in a round trip. Ch 12 Energy II: Potential energy

Ch.12 Energy II: Potential energy 3. The frictional force The total work done by frictional force is not zero in a round trip. See 动画库/力学夹/2-04功的计 算举例 (1)

Definition of conservative force One particle exerted by a force moves around a closed path and returns to its starting point If the total work done by the force during the round trip is zero, we call the force a conservative force, such as spring force and gravity If not the force is a nonconservative one Ch 12 Energy II: Potential energy

Ch.12 Energy II: Potential energy Definition of conservative force: One particle exerted by a force moves around a closed path and returns to its starting point. If the total work done by the force during the round trip is zero, we call the force ‘a conservative force’，such as spring force and gravity. If not, the force is a nonconservative one

Two Mathematical statements If F is a conservative force, we have A rab/+Wb 0 Fig 12-4 a Fds+|.Fds=0(121) Path1 Path2 F·ds=0 Statement 1 (b) b Fds=F·ds(123) Path 1 Path2 Path2 F·ds=|F· d Statement2 Path 1 Path2 Ch 12 Energy II: Potential energy

Ch.12 Energy II: Potential energy Two Mathematical statements: a a b 1 b 2 2 1 Fig 12-4 (a) (b) If is a conservative force, we have: → F W W 0 ab.1 + ba.2 = F d s F d s 0 a b b a  +  = → → → →   (12-1) Path1 Path2 Statement 1 → → → → → →  = −  =     F d s F d s F d s b a a b b a Path1 Path2 Path2 (12-3) 0  F  ds =   Statement 2 → → → →  =    F d s F d s b a b a Path1 Path2

Note: Newtons third law To every action, there is an equal and opposite reaction (1) Both the action and reaction forces belong to the system (2) The total work done by action and reaction forces is independent of the reference frame chosen(even in non-inertial frame) Prove point(2) Ch 12 Energy II: Potential energy

Ch.12 Energy II: Potential energy To every action, there is an equal and opposite reaction. Newton’s third law Note: (1) Both the action and reaction forces belong to the system. (2) The total work done by action and reaction forces is independent of the reference frame chosen (even in non-inertial frame). Prove point (2):

In s frame y W=W+2=「+ m m In s frame with velocity of vs relative to s frame X W=W+W2=f·c+f2·Chz (1-,)+|2(2-,n) ∫子+1的了的,山 =「f·+· W Ch 12 Energy II: Potential energy

Ch.12 Energy II: Potential energy 1 f  2 f  Ｏ 1 r  2 r  z y x m1 1 2 1 1 2 2 m2 W W W f dr f dr     = + =  +    In S frame: S In S’ frame with velocity of relative to S frame: s s v '  W' W' W ' f dr' f dr' 1 2 1 1 2 2     = + =  +    ( ) ( ) 1 1 2 2 f dr v dt f dr v dt s's s's       =  − +  −   f dr f dr (f f ) v dt s's        =  +  − +   1 1 2 2  1 2 1 2 f f    = − 1 1 2 2 f dr f dr     =  +   =W

12-2 Potential energy y 1, Definition 62 When work is done in a system(such as ball and arth) by a conservative force, the configuration of its rts changes, and so the potential energy changes from its initial u. value to its final value u We define the change in potential energy associated with the conservative force as A=-U1=-=「F(124) Ch 12 Energy II: Potential energy

Ch.12 Energy II: Potential energy 12-2 Potential energy 1.Definition When work is done in a system (such as ball and earth) by a conservative force, the configuration of its parts changes, and so the potential energy changes from its initial value to its final value . We define the change in potential energy associated with the conservative force as: Ui U f  ΔU =U −U = −W = − F ds f i   (12-4)