Box2D: Asteroids-Like Spaceship Physics

Box2D is massive overkill for Asteroids.

Without Box2D, assuming you're forcing yourself to use that for some reason, you just need some basic Newtonian physics.

Velocity is your speed in a particular direction. Acceleration is your change in velocity in some particular direction.

You can take your heading (rotation), generate from that a unit vector (cos(theta), sin(theta)) and then set your acceleration to that vector +/- a particular amount depending on whether up or down is pressed.

You can then integrate your new position and velocity each frame with simple Euler integration:

velocity += dt * acceleration position += dt * velocity 

For rotating, just modify your heading angle when you press left or right directly.

You now have the same 'physics' Asteroids has. Now you just need to add in collision detection so asteroids kill your ship and your bullets kill asteroids. Simple circle-circle collision can work just fine here.

check_collision(center1, radius1, center2, radius2): distance = square_root(center1.x * center2.x + center1.y * center2.y) if distance < radius1 + radius2: collide else no collide 

And there you have it. Add random Asteroids spawning, lives, and bullets, and you have reimplemented Astroids exactly.

Even if you want to keep using Box2D, to use it well you need to understand Newtonian dynamics, impulses and forces, what constraints are in theory, linear and rotational dynamics, etc. It's not hard to learn it all, but you will need to take some time reading up on it (it's not really feasible to fit it all in a single SE answer). Having had introductory high school or college physics will come in extremely handy.