I am taking AP physics and have taken physics before but am puzzled. I started thinking about the classical tug of war question. I get that a team wins in tug of war not because the pull on each other but because they push on the ground. This is b/c of newton's 3rd and b/c there is a net force there is acceleration. But then I thought about why a bolder shot out of a big sling shot can knock over a wall. The boulder exerts a force on the wall, the wall exerts a force on the boulder. So there must be a net force but why is there a net force. B/c of acceleration / f = ma? If yes can you please explain it and not just say yes you're right. I think I'm right i just can't convince myself why this happens. I know its extremely conceptual but I'm just stuck.