F = ma When two player collide on the field, they interact, exerting forces on each other which are equal in magnitude and opposite in direction.  The linear momentum of each player changes. momentum:   p = mv Since the forces are equal in magnitude and opposite in direction, the momentum changes are equal in magnitude and opposite in direction. The sum of the momenta or the total momentum of the two player does not change during the collision.

More details?

 When two player collide on the field, they interact, exerting forces on each other which are equal in magnitude and opposite in direction.  A net force acting on a player causes him to accelerate.  acceleration = change in velocity / time required to make that change or a = delta v / delta t For a given net force the acceleration of the player depends on his mass.  F = ma = m delta v / delta t In physics, we define the linear momentum p of an object as the product of the object's mass m times its velocity v. p = mv. Linear momentum is a measure of an object's translational (as opposed to rotational) motion.  It has magnitude and direction.  Its direction is the direction of the velocity.  If an object's velocity is changing, its linear momentum is changing.  We have for an object with constant mass delta p / delta t = m delta v / dt = ma = F. F = delta p / delta t is a different way of stating Newton's second law.  The rate at which a players momentum changes is equal to the force acting on the player.  If a force F acts on a player for a time delta t, then the change in the player's momentum id delta p = F * delta t. Collision times are usually very short, and during this short time interval the forces the players exert on each other are often much larger then all the other forces that are also acting on the players.  If we neglect all the other forces, we can make a statement about the sum of the momenta or the total momentum of the two player.  The total momentum does not change.  If the momentum of one player changes by a certain amount in one direction, then the momentum of the other player changes by the same amount in the opposite direction.  The sum of the changes is zero.

Before we analyze collisions on the field, let us look at some simple collisions in the lab.