How Much Force Does a Falling Boy Exert on a Net?

[Xept]

Hello

Homework Statement

I have this problem, even though it might seem very simple. A boy falls from a distance and lands on a net. When he reaches the net, it de-accelerates him with 4*g. The question is: What amount force does the boy exert the net?

The Attempt at a Solution

I would say that he simply exerts the force F = m*g, but some of my colleges say that he exerts the force F = m*4*g. I don't see how he should be able to exert this force (m*4*g), since gravity is the only force that exerts him. If he de-accelerates with 4*g, i would say that Fres =m*4*g is the netto force, and not the force he exerts on the net.

I hope this makes sense.
Thank you

 

[Doc Al]

Start by identifying all the forces acting on the boy. Draw a diagram showing those forces and the direction in which they point.

The force m*g (downward) is the force that gravity (the earth) exerts on the boy, not the force that the boy exerts on the net. If his acceleration is 4g (upward), then the net force on him is m*4g upward. Use that to figure out the force that the net exerts on him. (Don't confuse "net force" with the "force exerted by the net"!)