How i would start it:
(i dont know any of the vocab. of Force & Motion)
Dear (name of friend),
Our team won because of strength as well as team work. Even though you're my friend, I would not go easy on you. It's a contest and when it starts I will act like I don't know you. Good job to your team! ....
This is only an idea you can change it a bit and the vocab.
Hope it helped :)