* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Energy
Efficient energy use wikipedia , lookup
Dark energy wikipedia , lookup
William Flynn Martin wikipedia , lookup
Open energy system models wikipedia , lookup
Energy subsidies wikipedia , lookup
Energy storage wikipedia , lookup
100% renewable energy wikipedia , lookup
Low-Income Home Energy Assistance Program wikipedia , lookup
Public schemes for energy efficient refurbishment wikipedia , lookup
World energy consumption wikipedia , lookup
Low-carbon economy wikipedia , lookup
Alternative energy wikipedia , lookup
Zero-energy building wikipedia , lookup
Work (physics) wikipedia , lookup
Energy Charter Treaty wikipedia , lookup
International Energy Agency wikipedia , lookup
Life-cycle greenhouse-gas emissions of energy sources wikipedia , lookup
Potential energy wikipedia , lookup
Energy policy of the United Kingdom wikipedia , lookup
Distributed generation wikipedia , lookup
Regenerative brake wikipedia , lookup
Energy returned on energy invested wikipedia , lookup
Energy policy of Finland wikipedia , lookup
Energy efficiency in transport wikipedia , lookup
Energy harvesting wikipedia , lookup
Kinetic energy wikipedia , lookup
Internal energy 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
PREMEDICAL COURSE – PHYSICS, DR. EMŐKE BÓDIS WORK, ENERGY, CONSERVATION OF ENERGY, WORK-ENERGY THEOREM Work Mathematically, work can be expressed by the following equation: W = F • d • cos Θ where F is the force, d is the displacement, and Θ (theta) is defined as the angle between the force and the displacement vector. The force doesn't cause the displacement but rather hinders it. The SI unit of work is the joule. [ J = Nm = kg m2/s2 ] To gather an idea of it's meaning, consider the following scenarios: 1 The Meaning of Negative Work Force acts in the direction opposite the objects motion in order to slow it down. The negative of negative work refers to the numerical value that results when values of F, d and theta are substituted into the work equation. Cosine Θ is negative between 90 and 270 degrees. Energy In physics, energy is a property of objects, transferable among them via fundamental interactions, which can be converted in form but not created or destroyed. 1. Potential Energy An object can store energy as the result of its position. This stored energy of position is referred to as potential energy. 1.1. Gravitational Potential Energy The energy is stored as the result of the gravitational attraction of the Earth for the object. There is a direct relation between gravitational potential energy and the mass of an object. Epot = m.g.h To determine the gravitational potential energy of an object, a zero height position must first be arbitrarily assigned. Typically, the ground is considered to be a position of zero height. The SI unit of energy is the joule. [ J = Nm = kg m2/s2 ] 1.2. Elastic Potential Energy This energy is stored in elastic materials as the result of their stretching or compressing. Elastic potential energy can be stored in rubber bands, bungee chords, trampolines, springs, an arrow drawn into a bow, etc. A force is required to compress a spring: Fspring = k.x (Hooke's Law) Springs are a special instance of a device that can store elastic potential energy due to either compression or stretching. Espring = ½ k.x2 where k = spring constant If a spring is not stretched or compressed, then there is no elastic potential energy stored in it. The spring is said to be at its equilibrium position. The equilibrium position is the position that the spring naturally assumes when there is no force applied to it. 2 2. Kinetic Energy Kinetic energy is the energy of motion. An object that has motion - whether it is vertical or horizontal motion - has kinetic energy. Forms of kinetic energy: vibrational (the energy due to vibrational motion) rotational (the energy due to rotational motion) translational (the energy due to motion from one location to another) The amount of translational kinetic energy: Ekin = ½ m.v2 Kinetic energy is a scalar quantity; it does not have a direction. Unlike velocity, acceleration, force, and momentum, the kinetic energy of an object is completely described by magnitude alone. The SI unit of energy is the joule. [ J = Nm = kg m2/s2 ] Mechanical Energy When the work is done upon the object, that object gains energy. Mechanical energy is the energy that is possessed by an object due to its motion (kinetic energy = energy of motion) or due to its position (potential energy = stored energy of position). Mechanical Energy as the Ability to Do Work An object that possesses mechanical energy is able to do work. In fact, mechanical energy is often defined as the ability to do work. The Total Mechanical Energy The total amount of mechanical energy is merely the sum of the potential energy and the kinetic energy: Emech = Epot + Ekin The Principle of Conservation of Mechanical Energy If you know the kinetic and potential energies that act on an object, then you can calculate the mechanical energy of the object. Kinetic energy converted to potential energy and then back to kinetic energy. If there’s no friction (or another non-conservative force), then ME1 = ME2, or 3 These equations represent the principle of conservation of mechanical energy. The principle says that if the net work done by non-conservative forces is zero, the total mechanical energy of an object is conserved; that is, it doesn’t change. (If, on the other hand, friction or another non-conservative force is present, the difference between ME2 and ME1 is equal to the net work the nonconservative forces do: ME2 – ME1 = Wnc.) Kinetic Energy and the Work-Energy Theorem This is the complete Work-Energy theorem. It is powerfully simple, and gives us a direct relation between net work and kinetic energy. Stated verbally, the equation says that net work done by forces on a particle causes a change in the kinetic energy of the particle. 4