SECTION 3  WAVES

Peter W Jones MInstP

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Advice for young Physics Teachers.

Nature of Waves.

 

See OCR Gateway Science for OCR GCSE, Biology, Chemistry and Physics B, published by Collins.

Pages 196 and 197

 

Explaining to students the connection between this theory and the waves in sheltered inland  water that can capsize boats, and HGV bow waves that can cause caravan/trailer accidents, will greatly increase interest in the subject. It will only take a few minutes of teaching time to do this, but it will take much longer to decide how you will present the material. However, you will only have to do it once. I would have introduced this at least 15 years before I retired had I known about the Bath University research on caravans. !  15 years after I retired wave theory is still in the syllabus.

(Bath University Results: See www.caravanaccidents2.wordpress.com  paragraphs 9b and 9d. These are in “Archives for 2007″)

 

Figure 1  in the GCSE book showing interference patterns in water waves in a ripple tank is quite clear as it stands, but you will sink this topic before you have almost started if you do not demonstrate this in the laboratory.

Today, even in Birmingham, most pupils will have seen a boat proceeding in smooth water and making waves as depicted in Figure 5 right hand diagram.

If it is a very large boat it will make very large waves. A much.smaller boat nearby, if caught broadside on to these waves, is likely to be rolled right over ( ie capsized). If the occupants cast into the water are elderly there will be a considerable number of fatalities. This is a rare happening but always makes the news papers and TV.

The effect described takes place in comparatively shallow water  where adjacent land provides protection against the wind so that the water is quite flat. A related effect takes place as a result of a  tsunami. The earthquake that sets off the tsunami can be hundreds of miles from the place where the disaster is experienced. Ships and boats in the path of the resulting waves will hardly notice their passing, but as the waves approach shallower water the constriction of the waves into a smaller space results in the enormous and damaging waves.

The owner of the sailing cruiser shown below (photo “V”) came very close to disaster as on  rare occasions some very large motor yatchts coming into shallow water from the open sea will not realise that their bow wave is potentially dangerous to smaller boats. In this instance, even though the boat was under sail at the time, he managed to turn the boat through about 135 degrees so that it struck the waves at about 45 degrees to the crests and rode harmlessly over them. This option may not be open to the skipper of a pleasure boat carying around 100 passengers as it would need much more space and time to turn through 135 degrees.

Viewed in simple terms something similar may happen when an HGV/coach over takes a caravan/large high aspect trailer, but even if you could see the waves you could hardly take avoiding action on a motor way .

    Figure 5  (and the diagram below) shows constructive and destructive interference. If the top wave represents the caravan wave (if it is oscillating slightly) and the bottom wave the HGV wave, it can be seen that at any one time you may get slight snaking stopped by an over taking HGV, but on another occasion if the waves were in a different phase you could get the one wave reinforcing the other and finish up with a potentially dangerous snake.

 see also 

    www.schoolminibustrailers.wordpress.com