# Force applied to a net after dropping

1. Dec 8, 2014

### Toke

1. The problem statement, all variables and given/known data
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?

2. Relevant equations
F=m*a

3. 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?

2. Dec 8, 2014

### PhanthomJay

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.

3. Dec 8, 2014

### 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 $\Sigma F = ma$

4. Dec 8, 2014

### haruspex

To clarify, that is with the interpretation BvU offers:

5. Dec 8, 2014

### PhanthomJay

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