* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Conservation of Energy
William Flynn Martin wikipedia , lookup
Open energy system models wikipedia , lookup
Energy subsidies wikipedia , lookup
Potential energy wikipedia , lookup
Dark energy wikipedia , lookup
100% renewable energy wikipedia , lookup
Kinetic energy wikipedia , lookup
Energy storage wikipedia , lookup
Low-Income Home Energy Assistance Program wikipedia , lookup
Public schemes for energy efficient refurbishment wikipedia , lookup
Regenerative brake wikipedia , lookup
World energy consumption wikipedia , lookup
Zero-energy building wikipedia , lookup
Low-carbon economy wikipedia , lookup
Energy Charter Treaty wikipedia , lookup
Alternative energy wikipedia , lookup
Life-cycle greenhouse-gas emissions of energy sources wikipedia , lookup
Distributed generation wikipedia , lookup
International Energy Agency wikipedia , lookup
Energy policy of the United Kingdom wikipedia , lookup
Gibbs free energy wikipedia , lookup
Energy returned on energy invested wikipedia , lookup
Internal energy wikipedia , lookup
Energy policy of Finland wikipedia , lookup
Energy harvesting wikipedia , lookup
Energy efficiency in transport wikipedia , lookup
Energy in the United Kingdom wikipedia , lookup
Negawatt power wikipedia , lookup
Energy policy of the European Union wikipedia , lookup
United States energy law wikipedia , lookup
Energy efficiency in British housing wikipedia , lookup
Energy Independence and Security Act of 2007 wikipedia , lookup
Conservation of Mechanical Energy What do you think? • Imagine two students standing side by side at the top of a water slide. One steps off of the platform, falling directly into the water below. The other student goes down the slide. Assuming the slide is frictionless, which student strikes the water with a greater speed? – Explain your reasoning. • Would your answer change if the slide were not frictionless? If so, how? What do you think? • What is meant when scientists say a quantity is conserved? • Describe examples of quantities that are conserved. –Are they always conserved? If not, why? Mechanical Energy (ME) ME = KE + PEg + PEelastic Elastic Energy Gravitational 1 Kinetic 1 2 2 KE mv PE elastic kx Potential Potential2 2 PE gravity Hooke’s Law Energy Energy mgh Mechanical Energy (ME) • ME = KE + PEg + PEelastic –Does not include the many other types of energy, such as thermal energy, chemical potential energy, and others • ME is not a new form of energy. –Just a combination of KE and PE Classroom Practice Problems • A 1.00 kg book is dropped from a height of 2.00 m. Assume no air resistance. –Calculate the PE and the KE at the instant the book is released. • Answer: PE = 19.6 J, KE = 0 J Classroom Practice Problems • A 1.00 kg book is dropped from a height of 2.00 m. Assume no air resistance. –Calculate the KE and PE when the book has fallen 1.0 m. (Hint: you will need an equation from Chapter 2.) • Answer: PE = 9.81 J, KE = 9.81 J Classroom Practice Problems • A 1.00 kg book is dropped from a height of 2.00 m. Assume no air resistance. –Calculate the KE and PE just before the book hits the ground • Answer: PE = 0 J, KE = 19.6 J Table of Values for the Falling Book h (m) PE(J) KE(J) ME(J) 0 19.6 0 19.6 0.5 14.7 4.9 19.6 1.0 9.8 9.8 19.6 1.5 4.9 14.7 19.6 2.0 0 19.6 19.6 Conservation of Mechanical Energy MEi = MEf Initial ME = Final ME • The sum of KE and PE remains constant. • One type of energy changes into another type. – For the falling book, the PE of the book changed into KE as it fell. – As a ball rolls up a hill, KE is changed into PE. PEmax = mgh KE = 0 PE = mgh PE + KE = PEmax PE = mgh PE + KE = PEmax PE = 0 KE = PEmax Conservation of Energy • Acceleration does not have to be constant. • ME is not conserved if friction is present. –If friction is negligible, conservation of ME is reasonably accurate. • A pendulum as it swings back and forth a few times Conservation of Energy • Consider a child going down a slide with friction. –What happens to the ME as he slides down? • Answer: It is not conserved but, instead, becomes less and less. –What happens to the “lost” energy? • Answer: It is converted into nonmechanical energy (thermal energy). Classroom Practice Problems • A small 10.0 g ball is held to a slingshot that is stretched 6.0 cm. The spring constant is 2.0 102 N/m. –What is the elastic potential energy of the slingshot before release? Ans: 0.36 J Classroom Practice Problems • A small 10.0 g ball is held to a slingshot that is stretched 6.0 cm. The spring constant is 2.0 102 N/m. –What is the kinetic energy of the ball right after the slingshot is released? Ans: 0.36 J Classroom Practice Problems • A small 10.0 g ball is held to a slingshot that is stretched 6.0 cm. The spring constant is 2.0 102 N/m. –What is the ball’s speed at the instant it leaves the slingshot? Ans: 8.5 m/s Classroom Practice Problems • A small 10.0 g ball is held to a slingshot that is stretched 6.0 cm. The spring constant is 2.0 102 N/m. –How high does the ball rise if it is shot directly upward? Ans: 3.7 m Problem Solving Process • Draw a sketch • Identify: KEi, PEi, KEf, and PEf –Include both gravitational and elastic PE, as needed • Apply the equation: (KE + PE)i = (KE + PE)f • Solve for required quantity –Mass, speed, distance, height, etc. Practice Problem • Starting from rest, a child zooms down a frictionless slide from an initial height of 3.00m. What is her speed at the bottom of the slide? Assume she has a mass of 25.0 kg. Nonconservative Forces with Energy Considerations When nonconservative forces are present, the total mechanical energy of the system is not constant The work done by all nonconservative forces acting on parts of a system equals the change in the mechanical energy of the system Wnc Energy Nonconservative Forces and Energy In equation form: Wnc KEf KEi (PEi PEf ) or Wnc (KEf PEf ) (KEi PEi ) The energy can either cross a boundary or the energy is transformed into a form of non-mechanical energy such as thermal energy Transferring Energy By Work By applying a force Produces a displacement of the system Transferring Energy Heat The process of transferring heat by collisions between molecules For example, the spoon becomes hot because some of the KE of the molecules in the coffee is transferred to the molecules of the spoon as internal energy Transferring Energy Mechanical Waves A disturbance propagates through a medium Examples include sound, water, seismic Transferring Energy Electrical transmission Transfer by means of electrical current This is how energy enters any electrical device Transferring Energy Electromagnetic radiation Any form of electromagnetic waves Light, microwaves, radio waves Notes About Conservation of Energy We can neither create nor destroy energy Another way of saying energy is conserved If the total energy of the system does not remain constant, the energy must have crossed the boundary by some mechanism Applies to areas other than physics Now what do you think? • Imagine two students standing side by side at the top of a water slide. One steps off of the platform, falling directly into the water below. The other student goes down the slide. Assuming the slide is frictionless, which student strikes the water with a greater speed? – Explain your reasoning. • Would your answer change if the slide were not frictionless? If so, how? Now what do you think? • What is meant when scientists say a quantity is “conserved”? • Describe examples of quantities that are conserved. – Are they always conserved? If not, why?