物理课件动能定理(英文版)

1、Teacher: swallow GaoReview: Work = Force Displacement or W = FS Maker : gao yan.KE 1. definition : this is energy which something has because it is moving. 2. formula : KE = mv2 deriving a block of mass m is accelerated from velocity v1 to v2 by a resultant force F, while gaining this velocity ,its

2、displacement is s and acceleration is a . (see fig below)12v1v2as(Fig.1) w = FS=mas Maker : gao yanwhile v12v22 =2asso w = KE = mv22 mv12 (1)1212 mv2123. unit : joule (J) 4. note : w=FS= work done by the resultant force, not a component 5. Exmple : a horizontal force of 50N is applied to a cart . Th

3、e friction between the cart and the floor is 20N, if the cart has travelled 3m ,(see the diagram below) ,calculate: (a). The work done by the applied force F (b). The work done by the frictional force f (c). The gain in KE of the cart mv22 mv12 1212 Consider the body diagram Take the right as positi

4、veGiven: F=50N f=20N s=3mFind: WF , Wf , KEApply w = FS(a)WF =50N 3m =150J(b)Wf = 20N 3m = 60J negative work means that energy is lost in overcoming friction .(c) Apply w= mv22 mv121212FfcartKE= WF+ Wf =150J+(-60J)=90J. PE 1. definition : this is energy which something has because of its position ,s

5、hape or state 2. formula : PE= mgh 3. unit : joule (J) 4. note : PE=mgh=mgh1- mgh2 (PE is independent of the path ,just depends on the h1 -h2 . 1. A 5kg object is initially traveling alone a frictionless surface at a velocity of 2m.s-1, it is pushed by a 3n force in the direction of motion for a dis

6、tance of 4m. Whats the KE of the project after the push ? 2. A 2Kg mass moves across a rough surface with a frictional force of 2N at 4 m.s-1 , (a) The initial KE of the mass _ _V = 4 m.s-1 M = 2 kgFriction = 2N ( Fig. 4)Work done by friction on the block to bring it to rest = _. The friction does n

7、egative work. It acts in the _( same/opposite) direction to the direction of motion and tends to _( accelerate/decelerate) the block.(b)3. In a construction site ,a block with a mass of 50kg is raised slowly upward by a crane though a vertical distance of 3 m. (a). What is the work done by the crane ? (b). what is the gain in PE of the block ?4. A 2kg mass is pulled up by a string that passes over a smooth light pulley as shown opposite .if the hand mov