I think the point is to keep it simplistic.
I agree, but... "make things as simple as possible, but not simpler."
To stay on topic: There are two different scenarios after you tackle someone in football, from a physical point of view.
Either the bodies stick together after the collision (which is the case, most of the time, in football, e.g. inelastic collision) or they bounce off of each other (elastic collision).
Since the first case is much more common, here's all you need to know about it:
m1 * v1 + m2 * v2 = (m1 + m2) * vfinal
For example:
Before the collison:
Player 1 (mass = 80 kg) ---- (22 m/s) --> <-- (-4m/s)---- (mass = 120 kg) Player 2
----> positive x-Axis
After the collision:
vfinal = (m1*v1 + m2*v2) / (m1 + m2) = (80*22 + 120*-4) / (80 + 120) = 6.4 m/s (in positive x-Axis) -> Player 1 is overrunning Player 2.
--- (Player 1 + Player 2 = 200 kg)----- (6.4 m/s) --->
If vfinal is negative, Player 2 will overrun Player 1.
That's really as simple as it gets. Now you can calculate how fast a lighter Player has to run to stand a chance against a much heavier Player.
** had to change / to * before vfinal
