Force applied to a net after dropping

[Toke]

Homework Statement

Q: A boy falls with the acceleration 4g in a free fall until he hits a net. His mass is 78kg. You can ignore other forces. With what force does he influence the net?

Homework Equations

F=m*a

The Attempt at a Solution

I use the formula F=m*a which gives med 78kg*4*(9.82m/s²)=3,1kN. My teacher did though tell me that the answer were 3,9kN, but i can't figure out what formula to use then. What have i missed?

 

[PhanthomJay]

Homework Statement

Q: A boy falls with the acceleration 4g in a free fall until he hits a net. His mass is 78kg. You can ignore other forces. With what force does he influence the net?

Homework Equations

F=m*a

The Attempt at a Solution

I use the formula F=m*a which gives med 78kg*4*(9.82m/s²)=3,1kN. My teacher did though tell me that the answer were 3,9kN, but i can't figure out what formula to use then. What have i missed?

The 3100 N is the resultant force exerted on the boy by his weight and the net. The problem is looking for the average force exerted on the net by the boy.

 

[BvU]

Hello Toke, welcome to PF :)

As formulated in post #1, this is an impossible exercise.
First, it is strange that a free fall can take place with an acceleration 4g, but perhaps this is taking place on another planet.
Second, and more important: nothing is said about the net. Is it a steel grid like they use to reinforce concrete ?
Or is it made up of a lot of nice soft rubber bands ?

My guess is that the intention of the exercise was something like: a boy makes a free fall until he hits a net.
When he hits the net, his deceleration is 4g. What is the force he exercises on the net ?

If I'm guessing correctly, you can get the desired answer from a free body diagram or from $\Sigma F = ma$

 

[haruspex]

The 3100 N is the resultant force exerted on the boy by his weight and the net.

To clarify, that is with the interpretation BvU offers:

a boy makes a free fall until he hits a net.
When he hits the net, his deceleration is 4g. What is the force he exercises on the net ?

 

[PhanthomJay]

To clarify, that is with the interpretation BvU offers:

Right, and apparently the way teacher interprets it with the given answer. Not worded well for sure.