Momentum During a Tackle
After our football game against Lehigh University on Saturday, I started to think about all the physics that are involved in football. The most relevant that I thought about was about the recent discussion of the conservation of momentum and inelastic collisions. The equation (MAVA+MBVB=MAVA’+MBVB’) is especially applicable to a hit in football. There was one particular hit at the goal line where our player made a hit on the running back but the running back was able to still fall into the end zone. Our linebacker made the hit without running his feet so his velocity must have been close to zero, while the running back had a 5-yard head start running toward the end zone. The linebacker who made the hit is about 240 pounds (109kg) and the running back is 190 pounds (86kg). Say that the linebacker had a velocity of 0.5m/s when he made the hit and had been pushed into the end zone by the running back, while the running back was running into him at -5m/s (1m/s above the average running speed of an adult human). The running back was running in the negative direction in this situation. Since this was an inelastic collision, the running back and the linebacker were essentially stuck together and their momentum became a single entity. The final momentum of both players, according to the conservation of momentum equation was -1.9m/s.
This is why football coaches preach to keep your feet running during a tackle and to run them while you are being tackled. If the defensive player was able to make the tackle while running his, feet he would have more velocity going against the running back and would be able to stop him from reaching the end zone. Defensive coaches also teach their players to lift the running backs into the air when possible or take out the runner’s legs in order to stop the offensive player’s momentum.
It is interesting to think about all the other aspects of football that have so much to do with physics. Every pass has to do with constant acceleration due to gravity, velocity and projectile motion, while every block has to do with angles and momentum. The kickers and punters also deal with projectile motion. Not only football, but everything in life has something to do with physics. Whether it be driving a car, playing baseball, or using the elevator, physics is all around us.
Here is a link to an article that talks more about the physics of football!