Work is done when a force causes an object to move, and is calculated by multiplying the force by the displacement of the object in the direction of the force. Work is a scalar quantity measured in joules. Work is positive when the force and displacement are in the same direction, and negative when they are in opposite directions. No work is done when an object moves in a circular path, since the force is always perpendicular to the displacement. Energy exists in different forms and can be transformed from one to another, but cannot be created or destroyed according to the law of conservation of energy.