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In this example you're determining the speed of the car based on the wind flow on your hand. Putting the book in front might slow down the car, and it will probably also slow down the flow on the hand. However, if you still try to determine the speed of the car from the air flow on the hand, you'll probably be off, because car speed and wind flow aren't linked like that when the book is in the picture.

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It's a multivariate system, the dragforce is determined by the car geometry + book, hand, whatever.

They are both causes to speed.

In fact you don't even need flow to infer speed. You can just use pressure calculations and temperature, which is how airplanes measure their speed.

Controlling drag is a major component of the inputs to speed when flying an aircraft.

I can't tell if this is germane to the analogy or not, but the air flow over your hand, even behind a book, is still a function of the car's speed. As an illustrative example, imagine the feeling when the car is at 0 compared to when the car is at 60.

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