能量定义为物理系统执行工作的能力。但是，重要的是要记住，仅仅因为能量存在，这并不意味着它必须能够开展工作。能量以多种形式存在，例如热，动能或机械能，光，势能和电能。其他形式的能源可包括地热能和能源分类为可再生或不可再生能源。能量形式之间可能存在重叠，并且物体一次总是拥有多于一种类型。例如，摆动摆具有动能和势能，热能，并且（取决于其成分）可具有电能和磁能。根据能量守恒定律，系统的总能量保持不变，但能量可能转变为另一种形式。例如，两个台球碰撞可能会停下来，由此产生的能量在碰撞点变得健全并且可能有点热量。当球在运动时，它们具有动能。无论是运动还是静止，它们都具有潜在的能量，因为它们位于地面以上的桌子上。能量不能被创造，也不能被破坏，但它可以改变形式，也与质量有关。质量 – 能量等效理论指出在参照系中静止的物体具有静止能量。如果向物体提供额外的能量，它实际上会增加物体的质量。例如，如果加热钢轴承（增加热能），则会略微增加其质量。
Energy is defined as the capacity of a physical system to perform work. However, it’s important to keep in mind that just because energy exists, that doesn’t mean it’s necessarily available to do work. Energy exists in several forms such as heat, kinetic or mechanical energy, light, potential energy, and electrical energy. Other forms of energy may include geothermal energy and classification of energy as renewable or nonrenewable. There may be overlap between forms of energy and an object invariably possesses more than one type at a time. For example, a swinging pendulum has both kinetic and potential energy, thermal energy, and (depending on its composition) may have electrical and magnetic energy. According to the law of conservation of energy, the total energy of a system remains constant, though energy may transform into another form. Two billiard balls colliding, for example, may come to rest, with the resulting energy becoming sound and perhaps a bit of heat at the point of collision. When the balls are in motion, they have kinetic energy. Whether they are in motion or stationary, they also have potential energy because they are on a table above the ground. Energy cannot be created, nor destroyed, but it can change forms and is also related to mass. The mass-energy equivalence theory states an object at rest in a frame of reference has a rest energy. If additional energy is supplied to the object, it actually increases that object’s mass. For example, if you heat a steel bearing (adding thermal energy), you very slightly increase its mass.