I did a trial run of the experiment to see if it would work or not and I realised that the marble was a little big for the tube. So instead, I found cermic pie weights which are about half the size of a marble. That worked well in water. I am going to tie the tube to a metre ruler to keep the tube straight and then put it onto a slope and then I'll do the actual experiment from there. I tied rubber bands around the tube to the ruler, amrking off the start and finish point- the overall distance that the ball has to travel for each trial is 60 cm.