Same thing that happens when that little rock flies up off the road and contacts your windshield... Or a bullet impacts something at speed ....Only more so.
Reminds me of a Scifi murder story where a prof crated a beam that eliminated all momentum within it. He demonstrated with a pool table. Lined up his enemy took the shot. Everyone was shocked to see the ball leave a perfect circle as it left earth instantaneously. No one but the prof considered the relative motion of the earth, solar system, etc... His enemy was left with a perfect hole through his chest.... I need to find that short story again... Good read.