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1、ENERGY (KE & PE),Teacher: swallow Gao,Review:Work = Force Displacementor W = FSMaker : gao yan,.KE1. definition : this is energy which something has because it is moving.2. formula : KE = mv2derivinga block of mass m is accelerated from velocity v1 to v2 by a resultant force F, while gaining this ve
2、locity ,its displacement is s and acceleration is a . (see fig below),(Fig.1),w = FS=mas Maker : gao yan while v12v22 =2asso w = KE =,mv22 mv12 (1),mv2,unit : joule (J)4. note : w=FS=work done by the resultant force, not a component5. Exmple : a horizontal force of 50N is applied to a cart . The fri
3、ction 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,Consider the body diagram Take the right as positive Given: F=50N f=20N s
4、=3m Find: WF , Wf , KE,Apply w = FS WF =50N 3m =150J Wf = 20N 3m = 60J negative work means that energy is lost in overcoming friction . (c) Apply w=,KE= WF+ Wf =150J+(-60J)=90J . PE 1. definition : this is energy which something has because of its position ,shape or state2. formula : PE= mgh 3. unit
5、 : joule (J)4. note :,PE=mgh=mgh1- mgh2 (PE is independent of the path ,just depends on the h1 -h2,conclusion,.,.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 distance of 4m. Whats the KE of th
6、e 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 _ _,( Fig. 4),Work done by friction on the block to bring it to rest = _. The friction does negative work. It acts in the _( same/opposite) direction to the
7、direction of motion and tends to _( accelerate/decelerate) the block.,(b),3. In a construction site ,a block with a mass of 50kg israised 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 moves through a 0.5m in the shown direction. Whatsthe gain in PE of the block ?,Maker : gao yan,Bye-bye,Maker : gao yan,
