Answer:
The force needed is 400 Newtons.
Step-by-step explanation:
The force can be determined by means of Newton's law.
(1)
Where F is the force needed to stop the football player, m is the mass of the football player and a is the acceleration.

However, 1N = 1 kg.m/s^{2}
Hence, the force needed is 400 Newtons.