Buoyancy with Box2D on iPhone

Question

I was wondering if anyone can give me pointers on how to achieve the following using Box2D on the iphone:

1) I have a Box2D world with normal gravity of -9.8 2) The bottom half of the screen is a body of water

So when my sprite hits the body of water, I want him to react with buoyancy (similar to what's going on in this video: http://www.youtube.com/watch?v=0uX-1GXYIss)

Is the best way to achieve this to

1) simply calculate the y position of the main character sprite and then switch the gravity variable accordingly

or 2) is there a specific feature built into Box2D that will allow me to set my "water" sprite to behave as water in my world and "push" my main character sprite up (while still respecting the world gravity of 9.8)

Any info would be appreciated

Answer 1

The buoyant force is equal to the density of the fluid times the volume of the fluid displaced (which gives you the mass of the fluid displaced) times the acceleration due to gravity. The volume of fluid displaced can be costly to compute however. I would suggest making a simple estimate of the volume displaced based on the size of the object and how far it is submerged in the liquid.

Another very important force in fluid is the drag force. This is what makes it more difficult to move objects at high velocity through thick fluids. The drag force can easily be estimated by simply damping the velocity by some constant value: Force_drag = -b * v where b is your damping value and v is the object's velocity.