Corona SDK physics - apply force in particular direction?

Question

I'm new to Corona and as a learning exercise I'm creating a simple game of "keepie uppie" (http://en.wikipedia.org/wiki/Keepie_uppie) using the Corona SDK physics engine.

I have all aspects working correctly, except the ball motion on "kick". I'm using the touch event "began" phase to apply a kick, via the applyLenearImpulse method.

My issue is that the ball is generally behaving as if force is being applied from above, causing it to shoot towards the game environment's floor. This is despite the force being applied at the point of contact on the ball.

I have come up with the following workaround:

function ball:touch( event ) 

     -- only allow taps in bottom half of ball
     if ( event.y > ball.y and event.phase == "began" ) then

         -- temporarily move floor to just below ball
         floor.y = ball.y + ballSize

         local flipx = 0

         if(event.x > ball.x) then
             flipx = event.x - ballSize
         elseif(event.x < ball.x) then
             flipx = event.x + ballSize
         else
             flipx = event.x
         end

         ball:applyLinearImpulse( 0, kickForce, flipx, event.y)

     end
end

The above works by temporarily moving the floor position to just under the ball before the force is applied (the floor is then moved back to the correct position via an enterFrame event listener).

I also found that with this solution I had to flip the x position of the event touch, or it would otherwise bounce horizontally opposite to the expected direction.

The above is clearly not ideal. Should I be stopping the ball motion before applying the force and, if so, how? Or am I taking the wrong approach completely?

Answer 1

applyLinearImpulse is the command you want to use for this, but in all your workarounds you didn't mention the first adjustment I would try: reversing the direction of the force applied. The code you posted doesn't tell us the value of "kickForce"; have you tried negating that?