# Work Done on a System by an External Force

Work Done on a System by an External Force

We defined work as being energy transferred to or from an object by means of a force acting on the object. We can now extend that definition to an external force acting on a system of objects.

Work is energy transferred to or from a system by means of an external force acting on that system.

These transfers are like transfers of money to and from a bank account. If a system consists of a single particle or particle-like object, the work done on the system by a force can change only the kinetic energy of the system. The energy statement for such transfers is’ the work – kinetic energy theorem of that is, a single particle has only one energy account. called kinetic energy. External forces can transfer energy into or out of that account. If a system is more complicated.

Let us find energy statements for such systems by examining two basic situations, one that does not involve friction and one that does.