Force of water running through a hose

My daughter is doing a science experiment based on a launch loop (http://en.wikipedia.org/wiki/Launch_loop).



I hooked a 2600 psi pressure washer to two 1/8" OD tubes and then tethered them to a board. We rolled a tennis ball over the top of it when the water was flowing through the tubes (not just pressurized, but blasting out the end), but the tubes are not rigid when the water does not flow.



I understand the basics of the forces involved, but I don't know the details and have know idea how we can predict its performance. Can anyone help or point us in the right direction?



Thanks,

Michael