2.1 Systems

What counts as a system?

A system is any object or group of objects you choose to analyze. It can be a single particle or several objects treated as one unit, called a system of objects. Anything not included in the system is part of the surroundings, and interactions between them can be modeled as external forces acting on the system.

Point particles vs. extended systems

When rotation and internal structure don't matter, an object can be modeled as a point particle located at its center of mass, simplifying force analysis. For translational dynamics, treating a system this way lets us apply Newton's laws to the system's overall motion, ignoring internal forces between its parts.

Choosing your system wisely

The way you define a system affects which forces are internal (between objects within the system, canceling in pairs by Newton's third law) versus external (from outside the system, causing net acceleration). Choosing a system strategically — like grouping two blocks connected by a string — can simplify problem-solving by eliminating unknown internal forces like tension from the analysis.

Key terms

System

An object or group of objects chosen for analysis; everything else is the surroundings.

Surroundings

Everything outside the defined system that can exert external forces on it.

Internal force

A force between objects within the same system; these cancel in pairs (Newton's third law) when analyzing the system as a whole.

External force

A force exerted on the system by something in the surroundings; causes acceleration of the system's center of mass.

Point particle model

Treating an extended object as a single point located at its center of mass to simplify analysis of translational motion.

Try an AP-style question

Two blocks are pushed together across a floor. If both blocks are one system, which force is internal?

1. The push from your hand
2. The force each block exerts on the other
3. Earth's gravity on the blocks
4. Friction from the floor