How much force is needed to stop a 80 kg football player if he decelerates at 5m/s?

Use Newton's 2nd law of motion.

Here it is:

Force = (mass) x (acceleration)

= (80 kg) x (-5 m/s²)

= - 400 Newtons.

I called the acceleration negative because the player is slowing down.

The force comes out negative because it's in the direction opposite to

his motion.