Section+4.1+-+Work+Done+by+a+Constant+Force

**Work (W)**: The energy transferred to an object when the force acting on the object moves it through a distance.  Work is calculated by using the equation :

**W= (F cosθ)Δd** **Work** is a **scalar quantity.**

 The SI unit for work is (N.m). It is called Joule (J).

__**POSITIVE WORK:**__ If the force acting on the object and the displacement that results from it are in the same direction, positive work is done.

__**NEGATIVE WORK :**__ If the force acting on an object in motion is opposite in direction to the displacement, then negative work is done.

**__ZERO WORK:__** When the magnitude of displacement is zero, work done is zero. Also, if there is no applied force acting on the object, there is no work done. Finally, when the force and the displacement are perpendicular, **cosθ = 90,** work done is zero. Thus, no work is done by the force on the object, even if the object moves, and no energy is transferred to the object.

Ex.) a. Pushing against a tree, but it doesn't move. There is no displacement, no work done.  b. A paper airplane gliding after being thrown. No applied force is acting on the plane, no work done.  c. Lifting up a backpack, then walking horizontally forward. Normal force is perpendicular to displacement, no work done.

 Work is also **zero** when a **centripetal force** is acting on an object in **circular motion**.  This is because the orbit is considered to be a series of individual displacements perpendicular to the force on the object.