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TRACKS
Wheels are fine if you have a flat, even surface but if you've ever tried roller blading across an uneven surface (or even through mud!) you'll see that they don't work well in all situations. This is where tracks come in. Many vehicles - from tanks to snow mobiles use two sets of continuous, hinged, metal tracks instead of wheels. This distributes the weight of the vehicle over a much larger area so it doesn't sink into the mud or snow.
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 So why do wheels sink in muddy ground but tracks don't?
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 Tracks have a larger surface area which spreads the load. Therefore, there's more mud holding up the same weight and the pressure is less.
But what is pressure?
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Pressure tells us how concentrated a force is. It's how much force there is on a given area (usually a square metre) and it's usually measured in pascals.
1 pascal is equivalent to 1 newton on each square metre (1 N/m2).
There is another advantage to using tracks. They are in contact with a large area of the ground. A soft area of ground might swallow up a wheel and it would never get out. But the tracks might have one bit on the soft ground and another bit on some hard ground. The bit on hard ground retains its grip using friction.
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So what's friction?
It's a force between two surfaces.
Most of the time, we want to reduce friction as much as possible because it slows down anything that is moving and that's wasteful. However, sometimes it is a good thing - it gives us grip between two surfaces.
How does it do that?
On a microscopic scale two surfaces are never perfectly smooth. The rough edges make contact with each other.
The diagram shows a microscopic view of a shoe (top surface) pressing on the ground (lower surface).
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MOBILITY legs crawler tracks wheels
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