How would my mom react to me officially wanting to play football?

So I wanted to play football for a while now, and everyone I know except my mom says I would be good at it. A couple days ago my friend said I would be good on tackle (by the way I'm 15, 236lbs, 6ft). I wanted to play football because I enjoy watching the sport, the sport itself, I wanted to get more fit and I wanted to do something that would help me feel more masculine. But after telling my mom how my friend said I'd be good on tackle, she just let it at "You wouldn't lime it". I don't have a father so I feel she can't relate to me like a regular, loving father. My relationship with my mom is very good, and it's not like playing football in my local town could have any drawbacks -- I already have all A's & B's, and I know I'd be able to make at least one friend by playing football. She knows I'm cl 1 obese, so that a sport like this would be beneficial to my health. So if I told her I ABSOLUTELY wanted to sign up for tryouts in the fall, how would she react? And why is she the only person to say I both wouldn't be good at it AND wouldn't like it? How would a usual mother react to this, and why would she say this?