力的一般概念通常被视为以理想化的方式作用于物体。 （这对于科学，特别是物理学中的大多数事物来说实际上是常见的，因为我们创建理想化的模型来突出我们要特别注意的现象，并忽略我们合理可能的许多其他现象。）在这种理想化的方法中，如果我们如果一个力作用于一个物体，我们画一个箭头指示力的方向，并表现为力量全部发生在那一点。但实际上，事情并非如此简单。如果我用手推动杠杆，力实际上会分布在我的手上，并且推动杠杆分布在杠杆的那个区域。在这种情况下使事情变得更加复杂，力量几乎肯定不会均匀分布。这就是压力发挥作用的地方。物理学家应用压力的概念来识别力在表面区域上分布。虽然我们可以谈论各种背景下的压力，但在科学中讨论该概念的最早形式之一是考虑和分析气体。在热力学科学在1800年形式化之前，人们认识到加热时的气体会对包含它们的物体施加力或压力。 1700年代欧洲开始使用加热气体悬浮热气球，中国和其他文明在此之前就已经有了类似的发现。 1800年还看到了蒸汽机的出现（如相关图像所示），它利用锅炉内产生的压力产生机械运动，例如移动河船，火车或工厂织机所需的机械运动。这种压力得到了气体动力学理论的物理解释，其中科学家们意识到，如果一种气体含有各种各样的颗粒（分子），那么检测到的压力可以通过这些颗粒的平均运动来物理表征。这种方法解释了为什么压力与热和温度的概念密切相关，热量和温度的概念也被定义为使用动力学理论的粒子运动。对热力学感兴趣的一个特殊情况是等压过程，其是压力保持恒定的热力学反应。
The general concept of force is often treated as if it acts on an object in an idealized way. (This is actually common for most things in science, and particularly physics, as we create idealized models to highlight the phenomena we way to pay specific attention to and ignore as many other phenomena as we reasonably can.) In this idealized approach, if we say a force is acting on an object, we draw an arrow indicating the direction of the force, and act as if the force is all taking place at that point. In reality, though, things are never quite that simple. If I push on a lever with my hand, the force is actually distributed across my hand, and is pushing against the lever distributed across that area of the lever. To make things even more complicated in this situation, the force is almost certainly not distributed evenly. This is where pressure comes into play. Physicists apply the concept of pressure to recognize that a force is distributed over a surface area. Though we can talk about pressure in a variety of contexts, one of the earliest forms in which the concept came into discussion within science was in considering and analyzing gases. Well before the science of thermodynamics was formalized in the 1800’s, it was recognized that gases when heated applied a force or pressure onto the object that contained them. Heated gas was used for levitation of hot air balloons starting in Europe in the 1700’s, and the Chinese and other civilizations had made similar discoveries well before that. The 1800’s also saw the advent of the steam engine (as depicted in the associated image), which uses the pressure built up within a boiler to generate mechanical motion, such as that needed to move a riverboat, train, or factory loom. This pressure received its physical explanation with the kinetic theory of gases, in which scientists realized that if a gas contained a wide variety of particles (molecules), then the pressure detected could be represented physically by the average motion of those particles. This approach explains why pressure is closely related to the concepts of heat and temperature, which are also defined as motion of particles using the kinetic theory. One particular case of interest in thermodynamics is an isobaric process, which is a thermodynamic reaction where the pressure remains constant.