It depends on where it is applied and how focussed it is.
We know from Newton that Force = Mass times acceleration,
and we know that a whole body can survive 100 g acceleration for short durations
what speed would he be traveling at, gravity constant, how many Newtons would be in the impact
So if a big guy weighs 100 kg (I am a big guy) and 1 g is 9.80665 newtons of force per kilogram he might survive (briefly) 100* 100 * 9.80665 Newtons applied very briefly and evenly if in a car crash
Car seat - 98,000 Newtons won't kill;
Know a bullet from a famous handgun
http://en.wikipedia.org/wiki/9x19mm_Parabellum designed by Luger
weighs only 7.45 g and experiences 31,000 g to accelerate over its barrel length
So a force of only 31 * 7.45 * 9.80665 applied over the length of a barrel will get this small object moving fast enough to kill easily (assuming it slows down somewhere inside a head)
Bullet 2,000 Newtons will kill
View attachment 95017