Force applied to a net after dropping

  • Thread starter Toke
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  • #1
Toke
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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?
 

Answers and Replies

  • #2
PhanthomJay
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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.
 
  • #3
BvU
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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 ? :nb)

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
haruspex
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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 ?
 
  • #5
PhanthomJay
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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.
 

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