What Is eli5 what is a system in physics
Last updated: April 1, 2026
Key Facts
- A system is defined by drawing an imaginary boundary around the objects or area being studied
- Everything outside the system boundary is called the surroundings or environment
- Systems can be isolated (no interaction with surroundings), closed (energy but no matter exchange), or open (both energy and matter exchange)
- Choosing appropriate system boundaries is crucial for solving physics problems correctly
- The same collection of objects can be analyzed as different systems depending on where you draw the boundary
Understanding Systems in Physics
In physics, a system is a carefully defined portion of the universe that a scientist or student chooses to analyze. Think of it like drawing an imaginary circle or box around the things you want to study. Everything inside that boundary is your system, and everything outside is called the surroundings or environment. This clear separation is fundamental to solving physics problems effectively.
Why Systems Matter
Using systems helps physicists simplify complex situations. Instead of analyzing the entire universe, you can focus on just the relevant parts. For example, when studying a bouncing ball, you might define the system as just the ball itself, ignoring air resistance and the Earth. Or you might include both the ball and Earth in your system to analyze gravitational forces. The choice depends on what question you're trying to answer.
Types of Systems
Physicists classify systems into three categories:
- Isolated systems: No energy or matter enters or leaves (purely theoretical, doesn't truly exist in nature)
- Closed systems: Energy can be exchanged with surroundings, but matter cannot cross the boundary
- Open systems: Both energy and matter can be exchanged with the surroundings
System Boundaries and Choices
Selecting where to draw system boundaries is one of the most important decisions in physics problem-solving. A student analyzing a car might define the system as just the car's engine, the entire car, the car plus the road, or the car plus the Earth. Each choice creates different problems and requires different equations. The "correct" boundary depends on what forces, energy changes, and interactions are relevant to the specific question being asked.
Practical Examples
Consider a cup of hot coffee cooling on a table. You could define the system as just the coffee (open to heat exchange with air), the coffee plus the cup (closed system), or the coffee plus cup plus the room (effectively isolated if the room is well-insulated). Each definition makes different physics calculations relevant and simplifies the problem in different ways.
Related Questions
What is the difference between a system and surroundings?
A system is the specific part of the universe being studied with defined boundaries, while the surroundings (or environment) is everything outside those boundaries. The distinction allows physicists to focus on relevant interactions and ignore less important factors.
How do you determine system boundaries?
System boundaries are chosen based on what you want to analyze and what interactions are relevant to your problem. You should include all objects that significantly affect or are affected by the process you're studying, while excluding things that have negligible impact.
Why are isolated systems mostly theoretical?
Truly isolated systems don't exist in practice because objects always interact with their surroundings through gravity, electromagnetic forces, or other interactions. Physicists use isolated systems as simplified theoretical models to understand fundamental principles.
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Sources
- Wikipedia - Closed System CC-BY-SA-4.0
- Khan Academy - AP Physics 1 CC-BY-NC-SA-4.0