50 pounds for a 4'x8'x½" sheet of plywood?
I must be the Hulk, since I can comfortably carry 4 to 6 of them at a time. I'm pretty sure they only weigh about 15-20 pounds each. Even ¾" marine grade doesn't weigh more than 30-40 pounds a sheet. However, the awkwardness of carrying such a large item will surely make it seem to weigh twice as much
BTW, my experience comes from working in a lumber yard/home improvement center for over 5 years as a humper (stockman/loader). Entropy is right though, depending on the woods used, the weight will vary.
Just my 2¢