can you crank an ambulance up a sand dune?

Note: these articles have been published in InfoChem, the supliment to Education in Chemistry produced by The Royal Society of Chemistry.
Many are based on the two BBC OU TV series - Hollywood Science

Ice cold in Alex is a classic black and white film staring Sylvia Simes, John Mills and Anthony Quayle. Set in 1942 in the Libyan war zone, the three find them selves crossing a mined desert in an army ambulance trying to get back to the safety of Alexandra. Nearing the end of their journey they are forced to attempt to drive up a massive sand dune to escape the desert. Realising that the engine will simply burn out in the intense heat they do a cleaver thing. They remove the spark plugs from the engine so that the pistons no longer limit the motion of the engine, put the engine in lowest gear and use the crank handle to slowly crank the ambulance up the dune. After several tries they succeed allowing them to make it back safe to Alex. There is a classic scene towards the end of the film where they toast their safe arrival, downing several glasses of beer hence 'Ice old in Alex'! But can it be done?

In our Hollywood Science TV series Robert Llewellyn and I successfully cranked a car up a steep hill to test this stunt. Now the ambulance would have been much heaver but it would have had a greater gear ratio and so we felt confident that they could have done it - providing the slope of the dune was not too great.

So how steep is a sand dune? You might think that a sand dune could be any steepness from almost flat to a vertical cliff but actually this is not so. It's true that sandstone or chalk can produce near vertical cliffs as you can see in quarries or on the south coast such as Beachy head and the White cliffs of Dover. But this is because it's solid; sand however is more fluid.

Try the following experiment. Take a salt cellar and pour salt onto a flat table top. A little mound of salt will build-up and it looks like it will grow ever steeper to make a pile. Then all of a sudden you notice that the grains slip and the pile loses height to form a mound with a larger area base. As you continue to add more salt the mound grows higher and steeper but at a certain maximum angle we see our 'salt dune' slips.

What is true of our salt crystal model is also true for many different types of small particles including sand. Our little table top model gives us some insight into the maximum likely steepness of a sand dune - about 30-40 degrees. There is an experiment called a Hele-Shaw cell that is used to investigate this behaviour and is worth exploring if you want to know more about the science of flowing particles (see ref. below).

Now we know that there is a maximum angle beyond which the grains can't go it means we can be much more confident that our heroes could have cranked the ambulance up a sand dune. It would been a lot of hard work though!

[1] Ice Cold In Alex, 1958, Canal Image UK (video).
[2] For details of the Hele-Shaw cell see: SEED site: Hele-Shaw cell

How teachers can use these articles in a lesson

Why Hollywood Science

Open University Hollywood Science web site

Call for clips - do you have a film clip that needs investigating?

Jonathan would like to thank Robert Llewellyn, Gill Watson and Harry Kroto (Vega Trust), all the BBC teams, The Royal Society of Chemistry and all at the Open University.


Dr Jonathan Hare, The University of Sussex
Brighton, East Sussex. BN1 9QJ.

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