The Definition of Energy

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Energy and work occupy an important part of our ordinary life and are among the most important topics in physics. Work in terms of physics has quite a different meaning than the work we normally think of. In physics, work is done only when an object is moved in the direction of the applied force.

Energy in physics is defined as the ability to do work. That makes sense, right? After all, the more energy you have, the more schoolwork you can do...right. So anyway, another way to see it is that work is the force exerted multiplied by the distance, or W=FD. That also makes sense. If you push a really heavy object a certain distance, you do more work than if you push a lighter object that same distance.

The energy stored in the various sources covered in this page is, in one way or another, derived from the energy of the sun. For instance, when we burn wood, we are simply converting the sun's power that was stored in the growing plant into radiant (light) and thermal (heat) energy. Hydroelectric power is derived from the running water that flows from higher elevations to lower elevations. Again, it is the sun that provided the energy to evaporate water from lower heights and transported it to the upper heights as rain. So all the energy we deal with comes from the sun.

You've already read about kinetic and potential energy. Other types of energy that are discussed in this site are derivatives of kinetic energy. For instance, thermal energy is kinetic energy in the form of heat.


Work is defined as the scalar product of the force applied and the distance moved by the object.


If we consider work done by a non-consistent force moving an object from point s1 to s2, the equation for it would be,
Work and energy share the same unit and it's measured in the SI(International Standard) unit of joule(J) which is the product of Newton(unit for force) and meter. foot-pound is also commonly used in the US. And the relationship between the two units is,

Consider an object of mass m pulled with a force F with an inclination over a horizontal plane.

If we ignore the force of friction, the forces acting on the body would be the gravitational force: mg, the normal force, and the force applied. Since the normal force(straight upwards) and the gravitational force(straight downwards) are perpendicular to the direction of the object, the work done by the forces is equal to zero. In the same manner, the vertical component of the force applied does not do any work on the object since it's perpendicular to the direction of its motion. The only force that does work is the horizontal force of the object's kinetic energy.

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