ANY force at all, acting in the right direction, will stop him.
But smaller forces will take longer.
In order to either speed him up or slow him down at the rate
of 20 m/s every second, the force required is
(mass) x (acceleration) = (120 kg) x (20 m/s²)
= (2400 kg-m/s²) = 2400 N .
The force can either increase or decrease his speed at that rate,
depending on the direction in which it acts on him.
If the force is exerted opposite to the direction in which he's already
moving, then it can stop him if it continues long enough.
