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Work Power & Energy


Energy is defined as the ability to do work. Physicists classify energy into several types: kinetic, potential, heat, sound, radiant energy (e.g. light) and electrical, chemical and nuclear energy.

Kinetic energy is possessed by a moving object by virtue of its motion. It equals the work done to accelerate the object to a particular velocity or bring a moving object to rest. Then two principal forms of kinetic energy are known as translational and rotational. The first is due to motion in straight line while the second is due to motion in a circle.

Potential energy is due to the position of an object. It is not apparent until released. Two common types are gravitational potential energy and elastic potential energy. An object gains gravitational potential energy, as work is done to raise it against the force of gravity. When the object falls, it gains kinetic energy but loses potential energy. Elastic potential energy is gained as work is done to stretch or compress an elastic object such as a spring.

This type converted into kinetic energy when the spring is released and it regains its formal shape. An object possesses heat, or thermal energy by virtue of its temperature. It is, in fact, merely a form of kinetic energy, because the temperature of a substance depends on the motion of its component atoms or molecules; the higher its temperature, fester the molecules move. (Heat radiation is not, however classified as thermal energy but as radiant energy; it comprises the infrared part of the electromagnetic spectrum). Radiant energy consists of electromagnetic radiation and includes radio waves, visible light, ultraviolet and infrared radiation and X-rays.

Radiant energy is emitted when electrons within atoms fall from a higher to lower energy level and release the excess energy as radiation. Sound energy consists of moving waves of pressure in a medium such as air, water or metal. They consist of vibrations in the molecules of the medium, and sound can therefore be regarded as special form of kinetic energy.

Matter that has gained or lost some electric charge has electrical energy. It is a form of electrostatic potential energy. The movement of charges constitutes an electric current, which flows between two objects at different potentials when they are joined by a conductor, because the charges move from one object to the other object until an equal potential is restored to each. Chemical energy is possessed by substances that undergo a chemical reaction, such as combustion. It is stored in the chemical bonds between the atoms that make up the molecules of a substance.

During 1 reaction, the atoms of the reactants rearrange themselves to form different molecules of the products. If the products have less chemical energy than the reactants, energy is released during the reaction (in the form of heat, light or electrical energy, as in a battery). If on the other hand, the products have more chemical energy than the reactants, then energy is absorbed and is to be supplied to make the reaction possible. Nuclear energy is produced when the nuclei of atoms change, either by splitting apart or joining together.

The splitting process is known as nuclear fission, the joining together as nuclear fusion. Such changes can be accompanied by the release of enormous amount of energy in the form of heat, light, and radioactivity. The resulting motion of the nuclei and particles also cause an increase in the thermal and kinetic energy of its surroundings.


Work is done when a force makes an object move. It is defined as the product of the force and the distance through which the object moves. If the object moves in the same direction as that of the force then work is multiple of magnitude of the force and the distance moved. But if the force acts in a different direction to the movement of the object, then work done is equal to component of the force in the direction of movement (which is less than the total force) multiplied by the distance through which , the object moves. If a force acts in the opposite direction to the motion (such as a force applied to slow down a moving object), then work done is negative. Zero work is done when no motion results, as happens, for example, when someone holds up an object without moving it. Work is shown by the following equation. W = Fd cos0 Where W is the work done (in Joules), F is the force applied (in N), d is the distance moved by the object (in m) and 6 is the angle between the direction of movement and the direction of the applied force.


In everyday language, power means much the same thing as energy or work. However, in physics power is the rate of doing work. Power is measured in watts, one watt being equivalent to a rate of working of one joule per second. Thus, although it takes the same amount of work to lift 10 kg through 10 m in 30 seconds as it does to do it in 60 seconds, it involves twice as much power to perform the task in the shorter time. In another way, the amount of work that a machine can do depends both on its power and on the length of time for which it operates The power of a machine is given by: p= w/t Where P is the power (in W, watts), W is the work done (in j), and t is the time taken (in s) to do the work.

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