In connexion with work and energy, we shall adopt another approach in addition to treating objects as particles: the system model.


A system is a part of the Universe, isolated and treated as a unit when describing physical phenomena. In the system model, we focus our attention on a small portion of the Universe – the system – and ignore details of the rest of the Universe outside the system.

A valid system may be a single object or particle, a collection of objects or particles, a region of space and it may even vary in size and shape (such as a rubber ball, which deforms upon striking a

The system boundary is an imaginary surface that divides the Universe into the system and the environment surrounding the system.

An isolated system is a system which is not in interaction with its environment.

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