Bernoulli Effect While Driving
Examples of Bernoulli's Principle: Convertibles and Passing Traffic
A few examples of Bernoulli's principle to notice when driving down the road. Why convertible tops bulge and a car is tugged towards passing traffic.
Bernoulli's Principle
Bernoulli's principle, which results from conservation of energy, is a relationship between the elevation, pressure, and speed of a fluid. If there are no elevation changes, then as the speed of a fluid increases its pressure decreases. The faster a fluid is moving the lower its pressure, and the slower it is moving the higher its pressure. The fluid can be either a liquid or a gas. There are many examples of applications of this effect, including some that we can observe while driving.
Convertibles
When a sporty looking fast convertible, with the top closed, passes by at a high rate of speed, you might notice the cloth roof bulging upward. It looks as if some of the wind is getting caught under the top and trying to blow it off. However the convertible top can still bulge upwards even if the car is well sealed to prevent wind from getting under the top.
In reality, the wind is blowing over the top of the car. The high speed air outside of the roof is at a lower pressure than the still air inside the car. Hence because of Bernoulli's principle the cloth top of a convertible will bulge upward when the car is driving fast.
A cloth top on the back of an open truck can show the same effect.
Passing Traffic
When in a car that is stopped on the left turn lane of a highway, one can often feel the car being pulled towards a passing car or truck. To a lesser extent, when a car and a large truck pass each other closely on the highway, the car driver might feel the car being pulled towards the truck. Why?
When traffic passes near a stopped car, the motion causes a high wind speed between the stopped car and the passing vehicle. According to Bernoulli's principle, when the air is moving at a higher speed it has a lower pressure. So the high speed air between the vehicles, caused by the passing vehicle, is at a lower pressure that the air on the other side of the car, which is not moving. Hence the two stopped car is tugged towards the passing vehicle as a consequence of Bernoulli's principle.
The same effect can occur when a moving car and truck pass each other on the highway; however the effect may be less obvious because the air on the outside of each vehicle is also moving.
The driver of the car feels the effect more than the driver of the truck because the car is less massive than the truck. If the same force acts on two objects the less massive object will experience more acceleration than the more massive object. (That is why it is so hard to slide a massive object, like a refrigerator, across the kitchen floor.) So, when a car and truck pass on the highway, the car's driver might feel the car being pulled towards the truck, while the driver of the more massive truck does not feel the effect as much.
Bernoulli's principle works for both liquids and gasses, so two ships passing at sea can be pulled towards each other by a similar effect.
More Examples of Bernoulli's Principle
Bernoullis Principle and Storms
