Hi,
It's a problem of reference.
your touch are reference in Screen Space, but your AddForce understands only World space ( 3d or 2d).
so you'll need to convert touches position from ScreenSpace to WorldSpace, OR understand the realtion between the two if you want relative forces to be applied.
Explore this and get back to me if you have no success, I'll do a sample, this is an important concept to digest for almost everything you'll do with input interactions and 3d/2d content.
Bye,
Jean