While work done is indeed a scalar quantity, it can be assigned a positive or negative sign to indicate the direction and nature of the work being done. The sign convention for work done depends on the relationship between the force applied and the displacement of the object.
When a force and displacement are in the same direction (i.e., the force is applied in the same direction as the displacement), the work done is considered positive. This implies that energy is being transferred to the system or that work is being done on the object.
Conversely, when the force and displacement are in opposite directions, the work done is considered negative. In this case, work is being done by the system, and energy is being transferred out of it.
For example, if you push an object in the same direction it moves, you are exerting a force in the direction of motion, and positive work is being done on the object. However, if you apply a force opposite to the object's motion, such as pushing it against its direction of movement, the work done would be negative.
The sign convention for work allows us to account for the energy transfer and the nature of the interaction between the object and the applied force. By considering the signs of work, we can determine if energy is being added to the system or taken away from it.
It's important to note that the magnitude of work done, regardless of the sign, represents the amount of energy transferred or transformed. The sign simply indicates the direction of that energy transfer or work interaction.