You must have code else where that is causing this to happen.
This code works perfectly fine:
[lua]
local physics = require (“physics”)
physics.start()
physics.setGravity( 0, 9.8 )
local ground = display.newRect(0, 0, 1024,10)
ground.y = display.contentHeight/2
physics.addBody(ground, “static”, {friction = .5})
local ship = display.newRect(0,0,50,50)
ship.x = display.contentWidth/2
ship.y = 0
physics.addBody( ship, { density = 2.0, friction = 0.5, bounce = 0.5 } )
[/lua]
Edit: Also, this is helpful for when you are trying to work with physics objects:
physics.setDrawMode( “hybrid” ) – overlays collision outlines on normal Corona objects