![]() Indeed, the internal forces arise from several distinct mechanisms all acting simultaneously. So a fluid – gas or liquid – behaves like a continuous and even slightly sticky substance, and when it encounters a moving body, its impact is modified by the forces acting among the particles themselves. Rather, they’re entangled like the fibres in cotton wool, and where their paths meet, the collisions between them amount to a form of collective behaviour. It’s true that in a gas, the molecules dart about and collide with rigid objects in their way, but their paths are not independent. But in the light of what we know today about fluid behaviour, the reality is more subtle. The idea was first promoted by Isaac Newton, and at the time, it made sense. Scientists once believed that ‘drag’ was caused by fluid particles striking the front of a moving body like grains of sand. Where do these forces come from? Until recently, the mechanism wasn’t clear. ![]() Together with the frictional shear stress, the pressure of the air flowing over the body surface will resist its motion, try to lift it off the ground, and maybe push it to one side as well. The effect of the headwind on a saloon car is more powerful still. Occasionally when you’re outdoors on a windy day, a sudden gust will almost knock you over. ![]() ![]() F.1816 Drag, lift & directional stability.R.1416 Aerodynamics and the moving train.C.2015 A simple model for tyre deformation.G.1619 Hysteresis losses in rolling wheels. ![]()
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