How to convert the joules into force?
Joules are a unit of work, or energy, so there is no conversion factor which can be applied to give a force.
If you can give us some more detail about your question, we may be able to help.
Consider an member of X m long is hit by an mass of Y kg. the structure observes an energy of Z joules. How can now convert this energy into load or force?
Just figure out the velocity at impact and then decide on how fast it sows down, then use f=ma. Oh and make sure the strain energy =the input energy. You might wanto do a bit of research on impat loads these things tend o be non-trivial.
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