Use of energy or force in spring question

[frostchaos123]

Question is:
A balloon is 36m on top of a river, a bungee jumper has a unstretched cord of 25m attached to him and his weight is 700n

If he is to stop 4m above the river what should the force constant of the cord be?

My calculation is that since the cord can only extend 7m, using hooke's law I equate 700n = -k * 7 which gives 100. However I was told that the force method is not correct and I should consider energy at start and end points instead.

So my question why is the force reasoning incorrect and if there is a general way to tell how to choose, whether by force or by energy to solve a given problem?

 

[kjohnson]

That would be correct if the 700N man was statically loaded (i.e. wasn't dropped or falling). But since he is dropped from some height that person with the stiffness you calculated would overshoot and crash into the ground because he has inertia. It takes more force to stop something moving than to hold something up if that makes sense.

 

[frostchaos123]

I see. In this case would it be correct to say that force should not be used, because even though the forces cancel out each other, there is a residual velocity that continues downwards and crash the person into the ground?