Work Energy Ch 7 Kinetic energy and work

Work & Energy Ch 7 Kinetic energy and work Ch 8 Potential energy and conservation of energy

Work W is energy transferred to or from an object by means of a force acting on the object. Energy transferred to the object is positive work, and energy transferred from the object is negative work. • “work” is transferred energy • “doing work” is the act of “ transferring the energy ” • 力作『正功』，是什麼意思？力作『負功』，是什麼意 思？ • 人推牆壁，人有沒有作功？

Work & Energy • Energy – Kinetic energy – Potential energy – unit Work －kinetic energy theorem △K= Kf - Ki =W • Work 和『位能』沒關係？ – definition：p 142 – unit v 2 = v 02+2 axd 乘上 (1/2)mv 2 = W = F xd F x= m a x (1/2)m v 02+ maxd (一維) (7 -6) W = F d cosψ (三維) (7 -8) (constant force) K f - K i= W = F xd (7 -7)

Work done by a variable force (§ 7 -8) (7 -30) (7 -31) (7 -32)

(7 -32)

Work & Force work done by a constant force (§ 7 -5， § 7 -6) W = F xd work done by a variable force (§ 7 -7， § 7 -8) (一維) (7 -6) (7 -31) (7 -32) 例題 7 -9 (三維) (7 -8) (三維) 例題 7 -10

Work –Kinetic Theorem with a variable force (§ 7 -8) W= Kf – Ki =△K

Work done by a spring force (§ 7 -7) • The work done by a spring (Fig. 7 -11) – (7 -22) – (7 -23) – (7 -24) • sample problem 7 -7 ( x = 17 mm → x = -12 mm) F x 畫 F-x 關係圖

Work －kinetic energy theorem △K= Kf - Ki =W Potential energy and work (8 -18) Conservation of mechanical energy

power (§ 7 -9) • • • Definition unit Home Work : Ch 7, Problem 45: elevator Home Work : Ch 7, Problem 46：spring Galeileo Galilei (1564~1642) Issac Newton (1642~1727) Robert Hooke (1635~1703) James Watt (1736~1819 ) 1784 『發明』蒸氣機？ James Prescott Joule(1819~1889)