Okay, so I'm making a platformer, and I wanna know how I can make a arc'd jump easily. Like what Mario does in super Mario Bros 1. Any ideas on a simple way to accomplish this?
Interesting question; having taken Physics classes, I take this sort of thing for granted, when really it is an intriguing idea.
The key point is that X and Y movements are distinct from each other. An object in motion will sustain its X motion, and its Y motion will be affected by gravity at a rate which you set.
You should track the sprite's X and Y velocities. When standing still, the sprite has an X velocity of 0; when moving, it has a constant velocity. When touching the ground, the sprite has a Y velocity of 0. When the jump button is pressed, give the sprite a Y velocity.
Now every frame, reduce the Y velocity by some amount; this is your gravity magnitude. But if the sprite is touching ground, set the Y velocity to 0 so that the sprite doesn't move into the ground. And then modify the sprite's position by (velocity*time).
Here is a decent-looking Physics lesson which demonstrates these separated velocity components: http://www.physicsclassroom.com/class/vectors/u3l2c.cfm
If this was not the answer you're looking for, and you actually mean something more complicated by an "arc'd jumping method", then please elaborate!
ExciteMike has some great examinations of the Super Meat Boy and Super Mario 3 jump physics on his blog: http://meyermike.com/wp/