This isn't a job for AE. It's a job for q real 3D application. You'd spend hours on end (maybe days on end) and drive yourself nuts trying to do it in AE.
I'm not sure I understand the scenario fully I'm afraid. The ball's flying into the goal, but it's already in the goal?
I'll assume it's not already in the goal and you want to see it flying through the air, hitting the net, dropping to the floor and the net rippling after the impact? Correct me if I'm wrong please!
You could try do it with the built in puppet tool, at least that's how I'd approach it.
Pin as any puppet tool pins into the net as you feel are necessary to get the deformation's you like, then use the PuppetTools 3 plugin (http://aescripts.com/puppettools/) to create a Null layer for each of those pins - it'll make it way easier to keyframe and animate.
Indeed, you might only need to animate one of those nulls. If you parent all the nulls to one 'master' one then use this expression in the Position for each one...
delay = .1;
...and set the delay for each one as appropriate to create a nice ripple effect, that could do the job nicely if it's just something quick in the background.
As for using Newton, you could give that a go too, but it could be tricker.
Do the same as above, setting up the net using the puppet tool and PuppetTools plugin, but run the simulation on some proxy layers that you'd need to create and parent each of the nulls PuppetTools creates to (because Newton doesn't see the nulls themselves).
If you do it correctly you should get the puppet tool setup controlled by the Newton simulation via the proxy layers you parented the PuppetTools nulls to. Make sense!?
let us know how you get on
There are a bunch of ways to do displace the net inside AE that may be easier and faster to do than a simulation in a 3D app. It all depends on the design. Don't discount using Displacement Map (Advanced Displacement Maps), Mesh Warp, Liquify, or many of the other displacement tools available in the standard AE Effects>Distort section. There's also FreeForm Pro and a bunch of other options. Like I said, it all depends on the design.
Thanks for the suggestions folks, I'll be working on it over the next few weeks so will let you know how I get on. I was even tempted to partially film it in miniature and then key/luma the net out and animate the ball separately (perhaps pulling the net with some thread or something. I'll give the puppet pin a shot first, then Displacement... Stylistically it's a cartoon illustration approach so not going for realism, and when I mentioned the ball being in the net already I was referring to cutting to this shot as the ball has already passed across the goal line, but the camera composition would be in a way that all you see is the ball flying in and the net stretching ( you wouldn't see the goal frame itself), and from side on.