>On the other hand, if the +apple runs into a brick wall at several km/sec, itâ(TM)s going to make a fair-sized hole. Where did the energy to break the bricks come from? You donâ(TM)t expect the wall to reform as the â"apple deals it a second blow, do you?
Nope, nothing of the kind, at most I would expect the bricks it knocks lose to land a tiny bit further away.
The +apple hits, transferring kinetic energy to the wall (it had to have a lot if it was moving at several km/h as in your hypotheses) - which knocks the bricks out and makes the hole.
Now what happens when the -apple hits depends on what the nature of the particle's are, more specifically whether they obey the Pauli exclusion principle. If not, it passes straight through the wall without breaking it at all (though the repelling between the particles as it passes through might cause some micro-cracks). This is the prevailing theory.
If it does obey the exclusion principle - then you have energy transfer just like with the +apple, and the +bricks move WITH the energy regardless of the source, so the bricks fall in the same direction - however because as they are knocked out they are ALSO repelled by the -apple's negative mass, they fall a few microns further than when the +apple hit.
At least, that's my understanding. I am not a physicist, just a fan of physics.