Check out this calculator of impact force.
It is good to show people, using numbers, how a 100kg person falling 4 m onto a 2m lanyard (assume 10% elongation) can result in an impact force of 19.6kN. That’s about 20 100kg persons jumping onto you! If we make reference to EN standard, the maximum impact force should not exceed 6kN.
Also found this link on rock climbing shock force and fall factor calculation.