I'm having a difficult time understanding why two billiard balls colliding is an isolated system, yet a car crashing into a wall is a non-isolated system. Does it really only have to deal with the fact that the car crash has friction as an "outside" force — but doesn't the pool table have friction? I'm unsure if what I'm saying makes any sense, but any clarifications on this would help me understand how to differentiate between these two types of systems. Thanks!
[Physics] Isolated and non-isolated systems: Momentum
classical-mechanicsmomentum
Best Answer
It all depends on what you want to study. The billiard balls are generally viewed as an isolated system for the purposes of explaining elastic collisions, but you could as well introduce friction with the pool table, and the consider the system balls+table as the isolated one. This just means you have to consider the friction.
In the case of the car hitting the wall, since the wall is grounded to Earth, as it is generally in this example, you cannot assume the system car+wall is isolated, however you could assume the system car+wall+Earth is. So when the car hits the wall, it is crashed as a result of the different deformation resistance it has w.r.t the wall. But if the wall were not grounded, and you consider the collision car-wall in say, space, the you could say car+wall is an isolated system.