One may think that 1,800 pounds of force exerted on the human body is enough to severely injure of kill; however that is the amount of force exerted on the bodies of two football players colliding. Staring with the most basic physics, the mechanics of the collision between the two players is described by the definition of a contact force, which is a force when two objects touch each other. Because the forces of the players not only have magnitudes but directions as well, the forces are considered vectors. This brings the discussion to the net forces. Because there is more than one force acting on the system between the two players and the football, the net force is a vector sum of all the forces acting on the system. The interesting thing about the collision in this story is that it involves three different objects, all of which have different force vectors. According to Newton's Second Law, the force vector is equal to the acceleration vector multiplied by mass of the object. The acceleration vector is the rate of change if the velocity and velocity is the rate of change of the position. This means that if the two players were traveling and the same speed and in this case, at 20 feet per second, the players would collide and both travel in the opposite direction at about 10 feet per second, this is without the consideration of the football whose small mass contributed slim to no effect on the collision.
As Fountain wrote in the article, "Momentum may have been conserved, but pride was not." Momentum is the change is collisions velocities, and thus is also described as being the difficulty of stopping a moving object. The momentum of the players are very similar, and the fact that it was found that about 1,800 pounds of force were exerted on the football players brings up a interesting question of how football players withstand such extreme forces. The answer: football pads and helmet. The creation of proper football equipment has enabled football players along with other athletes in contact sports to withstand these collisions with minimal injury or pain. The football pads cause the force of the collision to be spread over a longer period of time so that there is a smaller impulse on the body. The force is spread over a greater time to make the change in momentum much smaller. The reason why the ball was projected from the collision site towards the sidelines is because the vector force that it had during the collision was very minimal to the vector forces of the two players. The external forces of the players caused the momentum of the football to change and move away from the system at a much greater rate than the two players.
There are different types of collisions. The collision in which the football players were involved in was an elastic collision because the two players bounced off each other after colliding. The bouncy aspect is caused by the elasticity of the collision. Elastic energy is energy that is stored in a springy object when it is slightly deformed during a collision. Because there was no change in the total kinetic energy of the collision, momentum was conserved. Because the collision has two dimensions, both the x-component and y-component momentums need to be considered.
The Spiral
The trajectory is the term used to describe the path of an object. The projectile is the object and in this case is the football. The trajectory of the football during a throw is in the shape of a parabola as it leaves the quarterbacks hand to the receiver's hands. The angle of elevation is the angle of the initial velocity, and the force that is most exerted on the ball while it is in the air is gravity. The velocity of the football in the y-direction is initially high, but as it is projected into the air, the velocity constantly decreases until it reaches the top of the parabola and begins to drop. The velocity in the x-direction is constant because it is in a constant state of motion in the forward direction. When the football is not in a tight spiral, then the football has more air resistance. This causes the velocity of in the y-direction to decrease much more rapidly and in turn the football does not go as far.
There are several pictures that represent the type of collision that was discussed in the article. The first photo is a picture of a Denver Bronco's defender after hitting the Green Bay Packer's reciever and shows the ball being projected away from the collision. In this collision, the force of the Bronco's player was greater than the Packer's player becuase the Packer's player looks like he is being moved backward more so than the Bronco's player.
The next photo is a big time collision as well. You know when the helmet comes off and the football is sent flying that the collision had a lot of force involved.
Dr. Timothy Gay actually has his own website, and more importantly to this blog, he has a link to Football Physics. The link is shown below. Within this link, there are several videos that can be watched, along with class discussions about the physics of football.
http://physics.unl.edu/outreach/football.html
Have a good one!
Ryan
