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 January 28th, 2015, 10:50 PM #1 Senior Member     Joined: Jan 2012 Posts: 745 Thanks: 7 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
 January 29th, 2015, 01:32 AM #2 Senior Member   Joined: Apr 2014 From: Glasgow Posts: 2,164 Thanks: 736 Math Focus: Physics, mathematical modelling, numerical and computational solutions You divided it when you should have multiplied it. Total mass = mass per unit area $\displaystyle \times$ total area. Thanks from topsquark
January 29th, 2015, 04:34 AM   #3
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Quote:
 Originally Posted by Chikis 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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# a small box of mass 5kg is place on the turn table at various distance from the centre of a table.At a disatnce 5cm from the centre of the table the box is on the point of slipping when the rotates at 2rev/sec.find coeffient of friction between the box a

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