Two carts roll toward each other on a track and crash. Until now, every problem followed one object: one cart, one net force, one acceleration. A collision will not fit that frame. Two objects slam into each other, and what happens to one depends on the other.
Physics handles this with a choice. Instead of one object, you choose a set of objects and analyze the set together.
Here are the two carts again, just before the crash. A dashed boundary is drawn around them: the two carts, taken together, are one chosen set.
When we analyze the motion of a chosen set of objects together, we call that set the system.
The choice belongs to whoever does the analysis. For the crash, the useful choice is the pair: both carts in, everything else out.
Other choices are legal. One cart alone is a system too; so is a ball and the bat that strikes it.
Nothing joins a system by default. The track, the air, and the Earth stay outside unless you choose to put them in.
Two ice skaters glide toward each other and collide. For the collision, the two skaters are analyzed together as one system. Which objects are in that system?
Katya says: a system is a set of objects we choose, so for a two-cart crash we can take the two carts as the system and leave the track out. Is she correct?
A baseball bat strikes a ball. To analyze the strike, a student treats the bat and the ball together. Name the student's system.
The student chose to analyze the bat and the ball together.
A chosen set of objects analyzed together is called the system.
The system is the bat and the ball.
When we analyze the motion of a chosen set of objects together, what do we call that set?
A truck collides with a parked car. A student analyzes the truck and the car together. Which objects belong to the student's system?
Which statement about a system is correct?
Two pucks slide toward each other on a rink and collide. The chosen system is the two pucks. Which of the following is part of the system?