A periodontist is a dentist who specializes in the prevention, diagnosis, and treatment of periodontal disease, and in the placement of dental implants. Periodontists are also experts in the treatment of oral inflammation.
Ground speed. Ground speed is the horizontal speed of an aircraft relative to the ground. An aircraft heading vertically would have a ground speed of zero. Information displayed to passengers through the entertainment system often gives the aircraft ground speed rather than airspeed.
Hitting an immovable object, such as a wall, the speed of the car would have to be 10-15mph. I suppose the difference would be that hitting the wall would be instantaneous deceleration, whilst hitting another vehicle would give a cushioning effect. Its not really a case of How fast but more a case of how hard. For example an air bag will deploy in the case of a stationary car being hit by a lorry at 5mph.