You can also edit physical properties by right clicking on the part in the browser bar then clicking on properties. You will then see a window come up with about 5 or 6 tabs. I can't remember which tab it is off the top of my head, but it might be physical.
Anyway, then you hit update and it will show the physical properties for the part with regards to the material and the geometry of the part. You can manually edit these properties to achieve the correct weight, but I don't recomend this unless it is and unchangable part (e.g. battery, electrical stuff etc.) because after you change it once, it will never update again if you decide to change the material. However, this method is effective in finding cg and weight of already built parts, but
do not do this when designing parts.
The only problem with this system is that Inventor will not display weight only mass, volume, surface area etc. What I have done in the past is to measure the mass and calculate the weight.
I will leave you with one warning:
be carefull of units. For about 2 weeks I was giving my team the weight in Newtons.
They weren't too happy about that.