Weight of the gold plates
Joseph Smith claimed to have translated the Book of Mormon from gold plates that were shown to him by an angel. Based on the description of the plates, they would weigh 200 pounds (91 kg). However, this estimated weight contradicts several other pieces of testimony. This contradiction undermines the plausibility of Joseph Smith's claim that he found golden plates.
His description of the gold plates has several names in geometry: it can be called cuboid, right rectangular prism, rectangular box, rectangular hexahedron and right cuboid . This geometrical shape is the same of any book or shoe box.
We make the assumption that this geometrical shape is made of 100% gold. There is relationship between mass and volume called density. The density formula is d=m/v, and therefore m=dv: when d=density, m=mass and v=volume.
The volume of the rectangular prism is v=lwh, where l=length, w=thickness or width and h=height. The volume of the "golden plates" is equal to 6x6x8= 288 cubic inches.
One inch has 2.54 cm and a cubic inch has 2.543 or 16,39 cubic centimeters. So, the Smith's plates measures 288x16,39 cubic centimeter or 4720,32 cubic centimeters
The density of gold at room temperature is 19,3 g/cubic centimeter. And m=dv, the mass of the plates is 4720.32x19.3=91102,18 grams or 91,10 Kg. A pound is 0,453 kg so 91,10/0,453 equals 201 pounds (91 kg).
Using the wiki database you can conclude that he must be a very strong man. We must make the assumption that the plates are made of gold. However, he did the claim it once, but he always said that they were golden. In order to get a golden alloy with copper, silver and gold we have to have a composition of about 15%, 70% and 15% respectively. The weight of the plates lowers up to 107.56 pounds (49 kg). However, these plates look like the Sagan's "undetectable dragon in his garage" and their actual composition will be never known.
Smith claims that he ran, with the plates tucked under his arm through the woods, to his home.
- "As he was jumping over a log, a man sprang up from behind and gave him a heavy blow with a gun. Joseph turned around and knocked him to the ground, and then ran at the top of his speed. About half a mile further, he was attacked again in precisely the same way. He soon brought this one down also and ran on again, but before he got home, he was accosted the third time with a severe stroke with a gun. "
Smith's wife, Emma Smith, later recalled that she "occasionally lifted the plates, covered in cloth" to dust underneath.
Joseph Smith allowed others, including Martin Harris, to lift the box that the gold plates were supposedly in. Harris estimated the plates to weight 40 to 50 pounds. 
- Inch: http://en.wikipedia.org/wiki/Inch
- Volume of a cuboid: http://en.wikipedia.org/wiki/Volume#Volume_formulas
- Density: http://en.wikipedia.org/wiki/Density
- Gold: http://en.wikipedia.org/wiki/gold
- ↑ Ensign: The Ensign of the Church of Jesus Christ of Latter-Day Saints, Volume 31
- ↑ Martin Harris estimated the gold plates (which he did not see, but lifted the box that held them) to weigh 40 to 50 pounds (Ch 4, pp 50)
- Bill McKeever, How Heavy Were Those Gold Plates?