How does that account for finger size though? Children might have a hard press of the same area as my light press. Hm, could be they look at the rate of change in the growth of the tap event...but this wouldn't provide information about mass. I think it's a no go.
We need a force here. They know how heavy the iPad is. They have an acceleration. There's a force.
This is an interesting idea. If one wanted to attempt a mechanism like this in iOS, what APIs would be useful? I don't know UIGestureRecognizer & friends very well at all, but it looks like it wouldn't help when needing to measure contact-area at a given contact point. Any iOS programmers here that know?
No accelerometer needed -- flesh is fleshy and distorts under pressure.