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Physics Theory : Work & Energy

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Work is said to be done when a force causes a displacement in its own direction.
Mathematically, work is given by the product of the force and the distance through which the force acts.
∴ Work = Force × Distance
The SI unit of work is Joule, denoted by the symbol J.
1 Joule = 1 Newton × 1 meter or 1 J = 1 Nm.


Energy is the ability or capacity to do work.


Law of Conservation of Energy states that the sum of the total energy in the universe is a constant quantity.
Energy can neither be created nor destroyed.
Energy can only be converted from one form to another. Energy can exist in various forms such as mechanical energy, heat energy, light energy, sound energy, magnetic energy, electric energy, chemical energy and nuclear energy.
Renewable sources of energy include wind, water and solar energy.
Non-renewable sources of energy include coal and petroleum.


Potential energy is the energy possessed by a body due to its position or configuration.
Mathematically, potential energy is given by the product of the body weight (mg) and the vertical distance h through which the body is raised.
∴ Potential Energy = m g h
where m is mass of the body, g is the gravitational acceleration, and h is the height.


Kinetic energy is the energy acquired by a body due to its motion.
∴ Kinetic Energy = ½ m v2
where m is the mass of the body and v is its velocity.


Power is defined as the rate of doing work.
∴ Power = Work / Time
The SI unit of power is Watt, denoted by the symbol W.
1 Watt = 1 Joule/second or 1 W = 1 J/s.


Efficiency of a machine is defined as the ratio of the work output to the work input (expressed as a percentage).
∴ Efficiency of a machine = Work Output / Work Input × 100 %
Thus, the maximum efficiency any machine can have is 100 %.
The energy losses from a machine are mostly dissipated as heat (thermal energy).


Work, energy and heat are all measured in Joules (J).
Note that work done on a body results in a change in its energy, and heat is simply thermal energy.


Physics Quiz on Work & Energy.
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