Our high school physics course recently covered pressure. I invented the following scenario to illustrate something I don't understand about pressure. Say you have 2 thumbtacks/pins. They have the same mass. You press each pin to a wall with the same force, but only the one with the thin point moves and slides into a crack in the wall that it has created. So only that pin accelerated. F = m.a, so if F and m are the same for both pins, how come a isn't?