i plan on joining the Marines in a few years(when i graduate high school), and right now i am trying to get the best possible score for the PFT(physical fitness test). i am great with the pullups and the running part but i have trouble with the situps. the situp requirements are to lay on your back with you knees bent and you ankles touching your ass, and then you have your arms crossed over your stomach and when you situp, the forearms have to touch the thighs. i can only get my forerms to about 4-5 inches away from there. i do many situps and i have very hard abs(although they arent very well defined)...do i need to do weighted situps or something to get them stronger? or do they expect us to use momentum to get our forearms to the thighs?

Definition comes from a low body fat. If the exercise required you to simply swing your body upwards, you probably wouldn't be doing them in the navy.
I would suggest looking up some core strengthening exercises including ones you can do on the swiss ball. Adding weight to the equation would strengthen the area if you could complete proper repetitions.

How often are you trying to do these? If it is almost every day, you are probably overtraining your abs and they are just stagnant because of that.
you shouldn't be working them more than 3 times each week. That should be the ABsolute max. twice is fine. If you do add weight, don't do more than 2 times a week at first and see how you progress. You must rest in order to regenerate.

i do them monday, wednesday, and friday. as for body fat %, i have about 7%. and the Marines arent part of the navy.

anyways...thanks for the advice

lol, sorry bro im tired. No disrespect meant. monwedfri sounds good.
7% bf and you cannot see them or they are not well defined? perhaps they just havent developed as much as other parts of your body.

http://www.sendspace.com/file/rhcuo1

old..but kinda helpful...