I notice that the larger the truck the greater the magnitude of the lurch. Can anyone give a physical explanation to this?
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3 Answers
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Bernoulli's principle, the fast moving vehicle drags air with it creating a low pressure region. if you live in a country with high speed trains it's enough force to pull someone off a station platform if the trains don't slow down. (good explanation http://www.physics.umn.edu/outreach/pforce/circus/Bernoulli.html)
answered Apr 11, 2012 at 2:54
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1$\begingroup$ While correct, it is normal when stating causes and effects to call the force the cause and the velocity the effect. For some reason people don't follow this sensible convention regarding Bernoulli's principle. Velocity doesn't cause low pressure--- low pressure speeds air up as it enters. $\endgroup$ Commented Apr 11, 2012 at 7:21
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4$\begingroup$ The low pressure being the result of air being removed mechanically by being stuck to the vehicle and dragged away seems a clearer explanation than the low pressure causes the air to speed up to maintain some sort of integral $\endgroup$ Commented Apr 11, 2012 at 14:52
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The incoming truck and your car are moving, so they are displacing air out of their way. That means work is produced.
That also means that Bernoulli equation has nothing to do with this. Instead, you should consider the work- energy theorem if you want to understand the reason behind the lurch. Also you should consider reading how vortices are created.
The air that the truck pushes towards your car is the reason that your car is pushed. The turbulence and the subsequent low pressure at the rear of the truck is the reason your car is pulled.
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2$\begingroup$ "nothing to do with this"? Why not? Can you justify that statement? $\endgroup$ Commented May 22, 2020 at 19:13
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When the truck passes at high speed, that will increase the speed of the fluid around it. A pressure difference, between the truck and the car, will occur. So the car will be towed toward the truck. According to Bernoulli principal, it states that an increase in the speed of a fluid occurs simultaneously with a decrease in static pressure or a decrease in the fluid's potential.
