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January 28th, 2015, 10:50 PM   #1
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Calculate the mass

Calculate the mass, in kg, of a rectangular sheet of metal 15.7 cm long and 12.9 cm wide, if its mass is 38.1 g per cm2.
Area = $15.7\times12.9 =(202.53cm^2)$
202.53cm = $\frac{202.53}{38.1}$
= 5.31574803 g
= $\frac{5.3157}{1000}$
= 0.005316kg
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January 29th, 2015, 01:32 AM   #2
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You divided it when you should have multiplied it.

Total mass = mass per unit area $\displaystyle \times$ total area.
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January 29th, 2015, 04:34 AM   #3
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Quote:
Originally Posted by Chikis View Post
Calculate the mass, in kg, of a rectangular sheet of metal 15.7 cm long and 12.9 cm wide, if its mass is 38.1 g per cm2.
Area = $15.7\times12.9 =(202.53cm^2)$
202.53cm = $\frac{202.53}{38.1}$
For one thing, you shouldn't write "$202.53= \frac{202.53}{38.1}$.
That obviously isn't true! And, of course, as Benit13 said, you divided when you should have multiplied. You could have recognized that your self by looking at the units. Area is in $\displaystyle cm^2$, mass is in g, and its density (not mass) is in $\displaystyle \frac{g}{cm^2}$. To divide by a fraction, we "invert" the fraction and multiply: $\displaystyle (cm^2)(\frac{cm^2}{g})= \frac{cm^4}{g}$ will NOT give "g".

But multiplying gives $\displaystyle (cm^2)(\frac{g}{cm^2}= g$ because the "$\displaystyle cm^2$" terms cancel.

Quote:
= 5.31574803 g
= $\frac{5.3157}{1000}$
= 0.005316kg
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