Drag is a causitive input to speed though.

    nextCarSpeed(currentSpeed, wheelPower, dragForce, mass, deltaTime) =
    currentSpeed + ((wheelPower / currentSpeed - dragForce) / mass) * deltaTime

Increase "dragForce", and the resulting car speed decreases. That is a causal input, not an association.

▲ sigbottle 3 hours ago | parent | next [-]

Bertrand Russell objected to the notion of causation in the 1900s, because merely stating the updated dynamics of a system doesn't imply causation in any grand sense. Like hume, he dismisses causation, but not because of the problem of induction or anything, but because the concept seems incoherent to him. He especially emphasized this in physics - although maybe you can argue that for everyday human language, causation is good (Alice caused BCD to happen), in physics it doesn't belong.

Not that I entirely agree with his account but just some food for thought.

▲ djoldman 4 hours ago | parent | prev | next [-]

Initially I assumed it to be negligible but the numbers are actually very close!

I calculate around a 3% drop in speed (from 60mpg) for holding an average sized book out of the window. That's surprising to me.

It's not quite right to use hazard ratios to calculate life expectancy. But if we force it, it looks like being in the top 20% of "regular" sleepers compared to the bottom 20% confers 3-4.5 years of extra life (from birth, assuming everything else equal, assuming USA, etc.). That's 3.8%-5.7% more life (79 year life expectancy at birth in the USA as of 2025). So the numbers are actually close.

I made a bad analogy :)

But you get my point!

▲ ambicapter 3 hours ago | parent | prev [-]

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.

	▲	aeonik 3 hours ago \| parent \| next [-]
		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.
	▲	dwattttt 3 hours ago \| parent \| prev [-]
		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.