View source for Flywheel

[[File:spoked flywheel animation.gif|frame|right|Spoked flywheel]]
[[File:Leonardo-Flywheel-screenshot.jpg|thumb|180px|Simple flywheel in motion. Constructed based on drawings by [[Leonardo da Vinci]]]]
{{For|the film|Flywheel (film)}}
A '''flywheel''' is a heavy [[wikt:disk|disk]] or [[wheel]] that is attached to a [[rotation|rotating]] [[wikt:shaft|shaft]]. Flywheels are used for storage of [[kinetic energy]]. The [[momentum]] of the flywheel causes it to not change its [[rotation]]al speed easily. Because of this, flywheels help to keep the shaft rotating at the same speed. This helps when the [[torque]] applied to the shaft changes often. Uneven torque can change the speed of rotation. Because the flywheel resists changes in speed, it decreases the effects of uneven torque. [[Engine]]s which use [[piston]]s to provide power usually have uneven torque and use flywheels to fix this problem.

It takes energy to get a wheel (any wheel) to rotate. If there is little [[friction]] (good bearings) then it will keep rotating a long time. When energy is needed, it can be taken from the wheel again. So it is a simple mechanical means of storing energy. The amount of energy stored is a function of the [[weight]] and the [[speed]] of rotation - making a heavier wheel rotate faster takes more energy. Another factor is the radius (size) because the farther from the axis a part of the wheel is, the more energy it takes to make is rotate. These three factor can be represented by M (mass), <math>\omega</math> (angular velocity) and R (radius). Combining the two equations below gives <math>\omega</math><sup>2</sup>MR<sup>2</sup>/4. A fly-wheel is not just any wheel, but specifically designed to store energy. So it should be heavy and/or rotate fast. For example, some buses have a fly-wheel that is used for stopping and starting. When the bus stops (e.g. for a traffic light), the fly-wheel is connected to the wheels, so the rotational energy is transferred to it, so the bus will slow down while the fly-wheel speeds up. Then, when the bus has to start driving again, it is connected again and the energy is transferred back. Of course, you wouldn't want to lug a heavy wheel around on a bus, so it is made of a lighter material that can withstand extremely fast rotation. 

== Mathematics of flywheels ==
The [[kinetic energy]] of a rotating flywheel is

:<math>E = \frac{1}{2} I \omega^2</math>

Where the moment of inertia of center mass is equal to

:<math>I = \frac {1}{2} MR^2</math>

where <math>I</math> is the [[moment of inertia]] of the [[mass]] about the center of rotation and <math>\omega</math> ([[omega (letter)|omega]]) is the [[angular velocity]] in [[radian]] units. 

== History ==
The flywheel has been used since ancient times, the most common traditional example being the [[potter's wheel]]. In the [[Industrial Revolution]], [[James Watt]] contributed to the development of the flywheel in the [[steam engine]], and his contemporary [[James Pickard]] used a flywheel.

== Other meanings ==
In the world of [[venture capital]], the term "flywheel" is used to represent the recurrent, margin-generating heart of a [[business]].

[[Category:Energy]]
[[Category:Engineering]]