# Impact on fence structure - (momentum exchange)

1. May 7, 2009

### DaivdBender

Hello eveybody.

I am attempting to solve a complex problem in a simple manner. I am not a physicist so stick with me lol.

The Problem:

The problem is based around determining the maximum force of a object colliding into a fence like structure.

The object weighs 8kg and is travelling 27 m/s.

The object is expected to collide into the center of the fence ( I have my resons for this ) and be brought to a stop.

The center of the fence is constructed from 5.6 mm wire spaced 100x100mm and is welded to a 2m high 4 m wide frame. The area of impact is roughly 400mm high 300 mm wide.

The vertical post of the frame are cantileverd into a concrete barrier and are connected to a centeral pin which is connected to another barrier. ( they are all connected like a big chain and are cantileverd of the concrete barriers)

My solution

My idea was to treat the fence like structure as a spring. I built a computer model taking into account the structures propeties and loaded the fence incrementally to get the force deflection response of the fence under this loading condition.

Once i had done this it was possible to calculate the energy absorbed as a function of deflection and intern the maximum force exerted.

Assumptions

The collsion was inelastic
Energy was only lost in the collision due to the fences deflection energy and the momentum exchange to cause the inelastic collsions.

My Question ?

Im having a problem determing the energy loss in the momentum exchange.

Do i consider the whole fences mass, the localised area around the impact zone, the 5.6 mm wire inside the fence framing, or is there no clear answer in what to consider in momentum exchange ?

Does this sound like a reasonable approximate solution to my problem ?

I personally considerd only the interior part of the fences mass and neglected the framing and the adjacent fences.

Im just really stumped on how to determine the energy lost on impact.

Anyone have any ideas ?

And I cant do any experiments.