I never noticed in movies how people walked along the ridge/peak of the dune until I visited the great sand dunes in Colorado. Trying to get up the slope of the dunes I was sinking in almost thigh high, but once I got to the top it was sturdy. Why is it that the ridge of the dune can support weight but the slopes cannot?
In: Physics
Loose, particulate matter made of small hard objects [materials like sand, gravel, etc] will be able to hold your weight decently well if you just try to compress it directly down – it will disperse a little bit as you apply weight, but eventually, it will settle and support you just fine.
Particulate matter, however, is very weak when it comes to the amount of [*shear force*](https://upload.wikimedia.org/wikipedia/commons/thumb/a/a8/Simple_shear_in_2D.svg/1200px-Simple_shear_in_2D.svg.png) that it can endure, which is basically sliding between the layers of the sand.
In a solid like iron or wood, if you try to slide the layers like that, nothing will happen, because all the particles are bonded to each other so it resists that motion. But in something loose like sand, there’s nothing to stop the top layer of sand grains sliding and moving freely, because the grains aren’t stuck to each other.
So, when you’re just standing directly on top of a dune, with all of your weight applied downwards, the first case happens – the force is directed downwards and it can support the weight quite easily. But when you’re travelling up the side of a dune at an angle, each time you push off with your foot to try and move up, you’re making a shear force that drags the top layers of sand in the direction your foot is pushing, which is down the dune.
The easiest way to picture this is thinking about the fact that it’s perfectly easy to stand directly in the middle of a puddle, because your weight is applied directly downwards. But the second you try to push off with a foot to move, your foot will slip, because the shear force causes the water to slide with your foot, meaning you get no traction like you would if you were pushing against a hard solid.
In short, stuff like sand and gravel can support weight fine, but it can’t support you sliding across it, because there’s no attraction between the layers. When you’re travelling up the slope, you’re inducing a slipping force downwards, which makes your foot sink and dig in to the sand as you try to move up.
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