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Transcript
ATB - Work Review How much Work is done in lifting a 5 N book 3 meters? How much Work is done in carrying that same book up a flight of stairs that are 2 meters long and 3 meters high? Physics Chapter 7 ENERGY Mechanical Energy • Potential – Energy that is stored because of its position relative to something else. • Kinetic – Energy of motion Potential Energy Energy that is stored because of its position relative to something else. • Examples – A drawn bow – A stretched or compressed spring – Slingshot – Chemical energy – Water in an elevated reservoir – Ram of a pile driver Gravitational Potential Energy Potential Energy • Gravitational Potential Energy – PE due to an elevated position – Amount of PE is equal to the Work done in lifting it • W = Fd • The force (F) needed to lift it is the force equal and opposite to the objects weight Fw = mg • The distance (d) is the height, h above the Earth – PE = mgh – Only changes in PE are meaningful. That is, how much PE is converted into some other form. Potential Energy • It does not matter how the object gets to the height, h. The PE of the ball depends only on the height, not the path taken there. Whether it is lifted, pushed up a ramp, or carried up stairs, the PE is equal. Example #1 A 2000 kg car is picked up by a crane to a height of 10 m. A) What is the PE of the car? B) How much work was done by the crane? Kinetic Energy Energy of Motion • Examples – Pendulum – Roller Coaster Car – Any object that is in motion, i.e. has a velocity Kinetic Energy • Depends on… – Velocity – Mass – Kinetic Energy = ½ mass x velocity2 – Variable: KE – Unit: Joule – Equation: KE = ½ mv2 Example #2 A 6.0kg cat runs after a mouse at 10.0m/s. What is the cat’s kinetic energy?
