Popping the Head off a Zombie
Credit to Vsauce3 for the video.
Vsauce3 says between 5,000 to 15,000 N, the head will pop off.
Here we see Fred Jones Pop the Head off a Zombie.
A human head has a height of 23.9 cm for 99th percentile and 21.8 cm for 50th percentile.
Low End: 5,000 N * (0.218 m to 0.239 m) = 1,090 J to 1,195 J (9-C / Street Level)
High End: 15,000 N * (0.218 m to 0.239 m) = 3,270 J to 3,585 J (9-C / Street Level)
Results: AP: 5,000 N * 0.218 m = 1,090 J (9-C / Street Level) / LS: 5,000 N / 9.81 ~= 509.684 kg (Class 1)