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The SI unit of work is the joule (J), equal to a newton metre (N m). One joule is the work done when a force of 1 N produces a displacement of 1 m in the force’s direction. Using the same unit for work and energy emphasizes that doing work is a way to transfer energy.

The sign of work depends on the relative directions of force and displacement. Work is positive when force and displacement point the same way, and negative when the force opposes the displacement (a retarding effect). In lifting, the applied upward force does positive work on the load while gravity does negative work.

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Energy is the ability to do work—meaning to cause a change such as lifting, pushing, heating, or deforming an object. When one object does work on another, energy is transferred from the “doer” to the “receiver.” Energy and work use the same unit, the joule (J), because they measure the same kind of transfer.

Kinetic energy is the energy an object has because it is moving. A rolling ball, a moving bicycle, or wind can do work by pushing things or causing damage on impact. Motion matters a lot: a small increase in speed can make collisions much more severe because the moving object can transfer much more energy.

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Potential energy is stored energy that can be released later, often turning into motion. A book on a shelf can fall and do work; a stretched rubber band or compressed spring can snap back and launch an object. In these cases, energy is “held” by position or shape even when nothing is moving yet.

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