Physical object

A physical object is a collection of matter that exists within space and time. In both natural language and physical science, these are often called material objects or bodies. They are defined by having a boundary that separates what is inside from what is outside. This distinction is important because it allows us to identify a thing as a single unit. We can contrast these real things with abstract objects, like numbers, or mental objects, like ideas.

While a number like "3" does not exist in a specific place, a physical object like a banana or a table does.

To understand how an object works, we must look at its boundary and its movement. A boundary is a contiguous surface that tells us where an object ends. Some objects are rigid bodies, meaning their boundary stays the same shape as they move. Other objects are deformable bodies, which means their boundaries can change shape over time. In physics, we often observe objects moving as a single unit. They can move through translation, which is sliding from one place to another. They can also move through rotation, which is spinning around an axis.

Scientists also study the specific properties that every physical body possesses. These quantitative properties include mass, which is the amount of matter in the object. They also include momentum, electric charge, and energy. In the field of classical mechanics, these properties help describe how matter exists within a volume of three-dimensional space. This volume is often called the object's extension. In a different field called continuum mechanics, scientists view an object as a collection of smaller sub-objects. These tiny parts interact with one another through forces like pressure and mechanical stress.

One of the most interesting parts of studying objects is the concept of identity. Identity is the quality that allows us to recognize an object as being the same thing over time. This identity can stay the same even if the matter inside the object changes. For example, if you replace all the wheels on a car, it is still considered the same car. We use the simplest possible description to maintain this identity. However, identity has strict rules. If an object is broken into two pieces, only one of those pieces can keep the original identity.

Different branches of physics provide very different models of what an object actually is. In classical mechanics, objects are predictable collections of matter with clear positions. However, quantum mechanics changes this view significantly. In the quantum world, an object is often a particle or a collection of particles. These particles do not have a fixed physical position until they are measured. Instead, they are described by a probability distribution. This means scientists use math to describe the chance of finding a particle at a specific location.

At the smallest levels of science, even the definition of an object is debated. In particle physics, there is a discussion regarding elementary particles. Some scientists wonder if these particles are just points without any extension in space. Others look to string theory or M theory, which suggest particles might be extended in at least one dimension. This reaches into the very core of how we define the building blocks of the universe. Whether it is a large cloud or a tiny proton, the way we define an object changes based on the scale we are studying.

Finally, we can see how the study of physical objects connects to other fields like psychology and philosophy. In philosophy, a physical body is seen as an enduring object that follows a specific path through space and time. This is used to distinguish the physical world from the mental world of emotions or justice. In psychology, the study of objects varies by school of thought. Behaviorism focuses on objects and their properties as the only meaningful things to study. Meanwhile, cognitive psychology studies biological bodies to better understand how the mind works.