How Much Weight Can A 2x4 Hold?

I'm building a grow tent and it looks like one of the beams will have to hold 2 600w lights (with sealed hoods) and also a 50 lb carbon filter. I know nothing about the load bearing capacity of wood. Assuming the lights are 35 lbs each (I haven't received them yet) can a 2x4 safely hold 120 lbs?

how far is the 2x4 spanning or whats the lenght of your tent @ 8' it should hold about 200 lbs total

My tent is 10 feet long so I'm thinking of running a cross beam down the middle and nailing the 2x4's to each side of it. If I do this then each 2x4 will be about 5 feet long

mounted at each end a 2x4 can hold approx 250lbs if you bring the ends to 5' like you was sayin it will easily hold your needs!!

250 is pushin it ,keep the carbon filter to the side if possible or build a sling for it attach to something sturdy

Run the 2x4 vertically it will hold more.And use screws nails pull out alot easier.

You will be fine with the only being 5 foot long.
Thats 7 inches on each top plat of the wall it self.
So your only talking about a 4 1/2 foot spand.

You will increas the strength if you stand the 2x4 on end and not flat.

With your lights running in the oposite diection as your 2x4's you will be putting your hangers on two different 2x's which also drops the "load weight" down.

Yes I am a proffessional carpenter contractor and build custom grow rooms for a living.

You could also use hardwood 2x4's but forget about nails....Frame work is almost always made out of soft woods such as spruce/pine because it's cheap and easier to work with. However if you keep your beams short and balance the load as other have suggested it should work no matter what the wood. You could also consider geodesic dome technology because it is far superior to conventional carpentry.