Final answer:
If a force transfers energy to an object in the direction of its displacement, the work done is positive. If the force is in the opposite direction of the displacement, the work is negative.
Step-by-step explanation:
If a force transfers energy to an object and the force has a component in the same direction as the object's displacement, the work done by the force on the object is positive. Work is the product of the force applied to an object, the displacement of the object, and the cosine of the angle between the force and the displacement direction (W = Fd cos θ). When the force and displacement are in the same direction, the angle is 0° and cos 0° = 1, indicating positive work. However, if the force and displacement are in opposite directions, such as when the force acts upward and displacement is downward, the angle is 180° and cos 180° = -1, thus the work is negative.
As an example of positive work, consider pushing a shopping cart down the aisle where your push transfers energy to the cart causing it to move. In contrast, negative work is done when a generator applies a force opposite to the motion of an object, such as slowing down a briefcase's descent, thereby removing energy from the briefcase.
Work and energy have the same units and are measured in joules (J), where 1 J is equivalent to 1 newton-meter (1 N·m) or 1 kilogram meter squared per second squared (1 kg·m²/s²). The work-energy theorem connects the work done on an object to its change in kinetic energy.