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THE LAST WORD
Crunch timeWhy does freshly fallen snow squeak and creak when you step on it?
n The sound of feet on snow is an example of the stick-slip phenomenon. Other examples are squealing tyres and violin music.
When you try to drag one object over another the friction between them prevents movement. The objects won’t move at all until the dragging force at least matches the friction force. However, if they are elastic (and everything is a bit elastic), they will stretch. Then, when the elastic force matches the friction force, sliding starts.
For most materials, the friction force is higher when they are not moving than when they are. When movement starts, the friction force drops and the stretched elastic force will suddenly be too big, so the dragged object will accelerate as the stretch contracts.
The object’s momentum will carry it past the point at which the elastic force is less than the friction force, until eventually it stops again.
Now the higher static friction comes back into play and the objects stick together until the elastic force once again matches the higher static friction force.
Then the cycle repeats.The result is a saw-tooth
vibration of gradual stretching and sudden release – and this provides the creaking noise of snow. In reality, there is not a step transition between the two types of friction; the friction force just reduces very rapidly as relative movement starts.
There are a number of simultaneous microscopic phenomena that give rise to this reduction, such as the behaviour of any thin films of liquid between the surfaces. And, as rough surfaces start to slide, the collisions between their minuscule hills and dales tend to throw them apart slightly, reducing friction between them.
In the case of snow, I suspect that the principal cause of the sudden reduction in friction is the famous pressure-melting of ice that allows skaters to get such low-friction sliding. A build-up of pressure as the snow is elastically compressed underfoot finally causes a tiny bit of it to melt, suddenly reducing friction and allowing the stored elastic energy to dissipate in movement. As the movement reduces the elastic pressure, the ice refreezes, and the cycle starts again.
Not all materials behave in this way. It is virtually impossible to get polytetrafluoroethylene or PTFE (the coating on non-stick frying pans) to stick-slip because, unusually, its dynamic coefficient of friction is a little higher than its static one. But the next time
a squeaking knife on a plate sets your teeth on edge, you can blame the difference between static and dynamic friction. Adrian BowyerBath, Somerset, UK
n It was a very snowy start to the year all across Canada, so there are probably 35 million Canadians who can now answer this question.
From personal experience
of walking in fresh snow at -37 °C, I have found that you can tell the temperature from the sound of the squeak. The squeak gets higher in pitch as the temperature falls.
It is all about the shape of the snowflake clumps. On the Canadian prairies where it is extremely cold in January and February, the snow is often so light and uncompressed that you can blow it off your clothes after it has fallen on you. The density is so low that lifting a large shovelful to clear your driveway is not especially arduous.
Around the Great Lakes and Toronto, the air temperature remains higher and the snow clumps together much more easily. There, a shovelful is much heavier – heart attack-inducing unless you use a smaller shovel. In April and May there is also very wet snow on the prairies, so here
you must make sure that you only move a quarter of a shovel of snow at a time. All of this, I’m pretty certain, also explains why western Canadian snow shovels are wider than those from eastern Canada.
Another explanation for the squeak is that at low temperatures you crush the snow when you step on it, causing a squeak, rather than pressure-melt it. However, this doesn’t explain everything because the squeak disappears when the snow has remained at -37 °C for a long time. I have always considered this to be associated with the snowflakes subliming – evaporating without becoming a liquid – and becoming more rounded rather than pointy. If rounded, the flakes merge together very well without having to melt.
There is a very nice audio clip about this at bit.ly/ZwuSs8.Mike SmithUniversity of CalgaryAlberta, Canada
This week’s questionsWaTer sTonesHow do pebbles skim on water? Neither medium seems especially elastic, so how do the stones bounce?Juan BandiniVentimiglia, Italy
smelling DoubleWhy do we have two nostrils?Eleonore de BonnevalLondon, UK
“Walking in fresh snow at -37 °C, the squeak gets higher in pitch as the temperature falls”
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“at low temperatures you crush the snow you step on, causing a squeak, rather than pressure-melting it”
130511_R_LW.indd 149 2/5/13 11:50:36
