Conservation of Momentum and Dick Butkus.
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This is NFL Hall of Famer Dick Butkus, the greatest linebacker of all time. He played for the Chicago Bears from 1965 to 1973, during which time he had 1,020 tackles, 22 interceptions, and 27 fumble recoveries, and is renowned for his punishing tackles and speed.
In this picture, he is bringing down most of the Green Bay offense (which entire teams seem to be unable to do today as Rodgers and the Packers advance to 10-0.) There are 5 or 6 men, and each are linemen, so we'll say 250 pounds or 115 kg per person, and they're probably moving around 3 m/s as they push the pile. Butkus was listed at 6'3", 245 lbs, or 112 kg.
Using the Law of Conservation of Momentum (P before=P after), we can determine how fast he has to be moving in order to even match the momentum of the other men, and create a perfectly inelastic collision in which the the entire mass of 7 sweaty football players stops moving altogether.
m(total, offense)*v(total, offense) + m(Butkus)*v(Butkus) = m(Butkus and the unfortunate Packers)*v
(6 men *115 kg) * (3 m/s) + (112 kg) * v = 0 (final velocity is 0)
Butkus would have to be moving at 18.5 m/s, or about 40 miles per hour. The approximate world record for instantaneous velocity was set by Usain Bolt... and was 10 m/s.
However, the velocity that was found only measured the speed of his center of mass. Butkus's greatest strength was a player was his ability to explode his hips, chest, and arms through opponents, creating a velocity and therefore a momentum that was more than enough to stop anything short of a semi truck.
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