Under what circumstances should specific weight be used instead of density multiplied by gravity in fluid problems?

Specific weight is defined as the weight of a unit volume of a substance and is often expressed as the product of density and gravitational acceleration. It is a more convenient quantity to use in many fluid mechanics problems, especially in applications where weight is a primary focus.

In the context of fluid problems, specific weight is particularly useful in calculations involving buoyancy and pressure, where the distinction between weight and mass significantly affects the outcomes. Using specific weight directly allows for simpler calculations without needing to multiply density by gravity, which can be cumbersome and might introduce unnecessary steps or potential errors.

The choice of units can affect how specific weight is applied. In systems using the US customary units, specific weight is typically expressed in pounds per cubic foot (lb/ft³), which inherently incorporates gravity into the unit itself. This is especially useful in engineering applications involving weight, making calculations straightforward.

When operating in SI units, the concept of using specific weight may not directly correlate to the same ease of usage as in US units because the unit of specific weight in SI (newtons per cubic meter, N/m³) is derived from density multiplied by gravity. Thus, preference is often given to the straightforward use of density multiplied by gravity in these cases.

In summary, specific weight is most