ENERGY

What is Energy?

The simplest definition of energy is "the ability to do work". Energy is how things change and move. It's everywhere around us and takes all sorts of forms. It takes energy to cook food, to drive to school, and to jump in the air. 

What is gravity? 

Gravity is the mysterious force that makes everything fall down towards the Earth. But what is it? It turns out that all objects have gravity. It's just that some objects, like the Earth and the Sun, have a lot more gravity than others. How much gravity an object has depends on how big it is. To be specific, how much mass it has. It also depends on how close you are to the object. The closer you are, the stronger the gravity. 

What is force?
 In physics, force is a push or pull on an object. A force can cause an object to accelerate, slow down, remain in place, or change shape. 

What is work?
 We often use the word "work" in our everyday lives. For example, we would say that getting good grades in school takes a lot of hard "work". In physics, the term "work" has a specific meaning. Work, in physics, occurs when a force acts on an object to move it some distance from the start point (also called displacement). Work is calculated as the force times the distance.

Sources: http://www.ducksters.com/science/physics/work.php

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