Question

I want to make a moon rotating around a planet, or something along that analogy. So is there something I can call for physics.setGravity(0,0) that changes the position that the gravity pulls towards, particularly assigning that to be a physics body? If not, simply a specific x-y coordinate will be fine.

local moon = display.newImage ("moon.png")
physics.addBody(moon, {bounce=0, density=1.0})
local earth = display.newImage ("earth.png")
physics.addBody(earth, {bounce=0, density=1.0})

Thanks

Answer 1

Instead of playing around with the setGravity option, I think you should just rotate the images using something like this:

local physics = require("physics")
physics.start()
physics.setGravity(0, 0)
display.setStatusBar( display.HiddenStatusBar )

local background = display.newImage( "space.png", true )
background.x = display.contentWidth / 2
background.y = display.contentHeight / 2

local earth = display.newImage( "earth.png" )
earth.x = 100; earth.y = 200
earth.xScale = 0.7
earth.yScale = 0.7
physics.addBody( earth, {radius=40 } )
earth.bodyType = "static"

local moon = display.newImage( "moon.png", 40, 40)
moon.x = earth.x + 80
moon.y = earth.y
moon.xScale = 0.3
moon.yScale = 0.3
moon.rotation = 100  
physics.addBody( moon, {radius=10 } )
moon.bodyType = "dynamic"

myJoint = physics.newJoint( "pivot", moon, earth, earth.x, earth.y )

local orbit = function( event )    
        moon.rotation = moon.rotation + 1.5
end

Notice that I am not really mucking about with setGravity, but the speed of the rotation is defined in the orbit function.

The above code assumes you just have image files to represent your planetary bodies. If so, you will have to play around with the constants above (earth.x and earth.y, scaling values, etc) to make the whole system LOOK right for the images you choose.

Good luck!