The Balder train passes the northern part three times, a bit lower every time. If you measure the time for the train to pass, you will notice that if moves with approximately the same speed every time. You can use this to estimate the energy losses along the track. By counting the beams you can estimate the elevation difference. (Use the vertical distance 2.5m between the beams.)

Now look at the photo over the whole track and estimate how far the train has moved for every turn. (The whole track is 1070m - how many turns?)

• How much energy is lost for every turn? (Express in the form ΔE/mg)
• Can you express the energy loss in terms of an elevation loss per meter track (or per 100 m track)? This gives you an estimate of the friction coefficient.

The graph below shows a measurement of the elevation (obtained from the air pressure) during a tour in Balder.