A system is simply the collection of objects you've chosen to study. It could be a single block, two carts connected by a string, or an entire rocket. Everything outside the boundary you've drawn is the environment.
A system's properties — its total mass, its momentum, its energy — emerge from how its parts interact. The behavior of the whole system depends both on what's happening inside it and on how it interacts with everything outside its boundary.
Once you've drawn a system boundary, every force in the problem falls into one of two categories. Internal forces act between objects inside the system. External forces cross the boundary — they come from the environment.
Two skaters pushing off each other. If both skaters are part of your system, the push between them is internal — it cancels and doesn't move the system's overall center of mass.
Gravity pulling down on those same skaters. Gravity comes from outside the skater-skater system, so it's external — and it's the only thing that can move their combined center of mass.
This is a judgment call, not a rule. A system can be modeled as a single object when the internal details — how the parts interact, move relative to each other, or change shape — don't matter for the question being asked.
A useful test: if the parts of your system all move with the same velocity and acceleration at every moment, you can safely collapse it to one point. If parts slide relative to each other, stretch, or behave differently, you need to keep tracking them separately.
The center of mass is the point that represents the average position of all the mass in a system. For objects with symmetrical mass distributions — a uniform ring, a meter stick, a sphere — the center of mass sits right on the line of symmetry.
When objects aren't symmetrically arranged, you calculate the center of mass as a weighted average of position — heavier objects pull the center of mass toward themselves.
In plain terms: multiply each object's mass by its position, add those products together, then divide by the total mass. This is exactly what your infographic calls the "weighted average calculation."
Adjust each mass and position. Watch the center of mass shift toward the heavier object.
Notice the center of mass always sits closer to the heavier object — it's a weighted average, not a midpoint.
A 2 kg mass sits at x = 0 m and a 6 kg mass sits at x = 4 m. Find the center of mass of the system.
Three point masses sit on a line: m₁ = 2 kg at x = 0 m, m₂ = 3 kg at x = 2 m, m₃ = 5 kg at x = 4 m. Find the system's center of mass.