why in this question did the question answer only perform the product between area*P to calculate the mass?


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Shouldn't the mass be calculated as $m = (P \times A)/g$?

Considering that:

$P = F/A$, since: $F = m \times g$

$$P = (m \times g)/A$$

$$P \times A = (m \times g)$$

$$m = (P \times A)/g$$

I believe the result obtained from the dimensional analysis is incorrect since the unit of pressure is $lbf/in^2$, not just $lb$. Therefore, the correct unit obtained is $lbf$, not just $lb$.

My colleague said that if you know how many square inches there are, you can determine how many pounds are up there based on how many pounds are in each square inch, and it can be obtained by multiplying: area*p

But again, for me, the value obtained is in force unit and not in massa unit.

What am I not understanding?


1 Answer 1


It doesn’t matter. It is a bit sloppy, but it is understandable. Here on earth the weight of $1 \mathrm{\ lb}$ is $1 \mathrm{\ lb_f}$. So yes, if they wanted to be perfectly clear about it then they could have done a conversion with a conversion factor of $1$. That conversion should be understood even if it is not explicit.

  • 1
    $\begingroup$ I realized that the mistake I made was a technical one, and not related to the issue at hand. My assumption of using Newton's second law would only make sense if the pressure unit was in Pa or N/m^2. However, since the pressure value was given in lbf/in2, the value obtained by multiplying the area with P (in lbf/in2) is numerically equal to the mass value. $\endgroup$ Commented Apr 27 at 13:04

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