This joke relates to gravity because gravity is the force that keeps all people on the surface of the Earth. In other words, it keeps them "grounded."
My dad must be gravity because he makes sure that I'm always grounded.
This joke relates to friction because friction is the force that makes gears grind. On a deeper level, this terrible joke sparks a bit of "friction" between the two characters.
Do you want to know something that grinds my gears?
You're painfully unfunny. I don't think I want to be friends anymore.
This joke relates to elastic force because a slingshot uses elastic force to launch rocks or other such projectiles. Also, the character thinks that he can hit the target, but quickly realizes it is beyond his ability, or "a stretch."
Dang! I thought I could hit the target, but I guess it was a bit of a stretch
This joke relates to magnetism because the same side of two magnets repels. This means that if he is a chick magnet with the same side as all the girls, he actually repels girls, instead of drawing them in, as he intended to say.
I guess you could say I'm a bit of a chick magnet.
Yeah I guess... if you were the negative side along with the rest of the girls.
This joke relates to magnetic force because magnets are attracted to each other, and the boy is attracted to the girl. It also relates to friction, because the girl wants to slide away, but can't because the force of friction prevents her from doing so.
Oh god! I wish friction wasn't a thing so I could just slide away from him.
Dang, you must be a magnet, 'cause you're really attractive.
I could not think of a joke, but my excuse is that the elastic force from the springs in my keyboard became too strong for me to type, so I stopped trying.