Posted by Morgan on Wednesday, January 26, 2011 at 9:23am.
I have a rubber ball.
Its diameter is 5.16 cm.
Its density is 1.5 Mg/m^3.
I need to find its weight in US units.
I know to use w=pVg
I am coming out with the wrong answer though. Can someone help?

Physics  drwls, Wednesday, January 26, 2011 at 9:56am
1.5 Mg/m3 is the same as 1.5 g/cm^3
The volume is V = (pi/6)D^3 = 431.6 cm^3
The mass is 647.4 g
There are 454 g mass per lb mass, so that is 1.426 lb mass. That weighs 1.426 lb force when g = 9.81 m/s^2, the Earthsurface value.
Although most physics problems strongly advise you to use Newtons for weight in SI units or slugs for mass in British units, there is nothing wrong with converting to lb mass in this case, and then using 1 lbm > 1 lbforce. It saves you the trouble of dividing by g (for slugs) and multiplying by g (when doing W = mg to get weight in lb)
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