Energy

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Energy

This is a very informal definition of energy. For more precise information, visit Wikipedia.

Energy is often defined as the capacity of a physical system to do work. Energy can be kinetic or potential: kinetic energy is associated to the motion of a body and increases with the square of its velocity. The more kinetic energy a body has, the more work one has to do to stop it.

Potential energy is associated to forces or fields. For example, a book held above the ground has potential energy, since letting it go would let it acquire a significant velocity -hence, kinetic energy- before hitting the ground. Therefore, potential energy may be defined as the kinetic energy the body will acquire if let go.

Einstein showed that mass also contains energy: in fact, mass and energy are exactly the same thing. Therefore, a photon -which is pure energy- will be affected by a gravitational field; conversely, two massive particles such as an electron and a positron can be converted into pure radiation. A dramatic illustration of the mass-energy equivalence is the atomic bomb, where the mass lost by the uranium is converted into a massive high-energy blast. This is summarized by the popular formula E = mc2.

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