GTS autonomy
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Last edited by zachr; May 25, 2022 at 12:46 PM. Reason: physics lesson below!
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Acceleration is a major component of efficiency for city driving, but speed does matter quite a bit for highway miles where you're cruising at the same speed for an extended period. Remember that aerodynamic drag increases exponentially with speed. A leisurely cruise through the countryside at 50mph will be far more fuel efficient than a cannonball run at 100mph+ simply because you have to displace significantly more air to maintain that pace.
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Acceleration is a major component of efficiency for city driving, but speed does matter quite a bit for highway miles where you're cruising at the same speed for an extended period. Remember that aerodynamic drag increases exponentially with speed. A leisurely cruise through the countryside at 50mph will be far more fuel efficient than a cannonball run at 100mph+ simply because you have to displace significantly more air to maintain that pace.
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Displacement is really more about distance. If a car travels 50 miles, it displaces a certain amount of air. The amount is the same regardless of the speed. 50mph or 100mph, the cars displace the same air over that 50 miles. Yet, the car traveling 100mph has more drag and a lower mpg over that 50 miles. So, the answer is not quantity of displacement.
What about the rate of displacement? That goes up with speed, right? Well, yes, but so does the rate of distance (as that is what speed is) and they increase proportionally. So, while this will increase the power required, the power is used for proportionally less time for a given distance, so the energy/distance is the same. Still no change in force (or mpg, which is the inverse of force).
So, why then does the force from drag rise at all let alone by the square of velocity? Well, in order to displace the air you have to move the air out of the way. And that is the key. Moving the air requires giving the air kinetic energy. How much? Well that depends on how fast you are going. Twice the car speed means the air has to move twice as fast. And kinetic energy increases with the square of the velocity. E= 1/2mv^2.
So, drag increases by the square of the velocity not because you are displacing more air or because you are displacing air at a faster rate, but rather because you have to impart exponentially more kinetic energy to the air to move it at the higher speed to cause the displacement.
At least, I think. Im not a physicist.
Last edited by jhenson29; May 24, 2022 at 09:56 PM.
Drifting
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Agree that drag increases with the square of velocity, but I dont think its because more air is displaced.
Displacement is really more about distance. If a car travels 50 miles, it displaces a certain amount of air. The amount is the same regardless of the speed. 50mph or 100mph, the cars displace the same air over that 50 miles. Yet, the car traveling 100mph has more drag and a lower mpg over that 50 miles. So, the answer is not quantity of displacement.
What about the rate of displacement? That goes up with speed, right? Well, yes, but so does the rate of distance (as that is what speed is) and they increase proportionally. So, while this will increase the power required, the power is used for proportionally less time for a given distance, so the energy/distance is the same. Still no change in force (or mpg, which is the inverse of force).
So, why then does the force from drag rise at all let alone by the square of velocity? Well, in order to displace the air you have to move the air out of the way. And that is the key. Moving the air requires giving the air kinetic energy. How much? Well that depends on how fast you are going. Twice the car speed means the air has to move twice as fast. And kinetic energy increases with the square of the velocity. E= 1/2mv^2.
So, drag increases by the square of the velocity not because you are displacing more air or because you are displacing air at a faster rate, but rather because you have to impart exponentially more kinetic energy to the air to move it at the higher speed to cause the displacement.
At least, I think. Im not a physicist.
Displacement is really more about distance. If a car travels 50 miles, it displaces a certain amount of air. The amount is the same regardless of the speed. 50mph or 100mph, the cars displace the same air over that 50 miles. Yet, the car traveling 100mph has more drag and a lower mpg over that 50 miles. So, the answer is not quantity of displacement.
What about the rate of displacement? That goes up with speed, right? Well, yes, but so does the rate of distance (as that is what speed is) and they increase proportionally. So, while this will increase the power required, the power is used for proportionally less time for a given distance, so the energy/distance is the same. Still no change in force (or mpg, which is the inverse of force).
So, why then does the force from drag rise at all let alone by the square of velocity? Well, in order to displace the air you have to move the air out of the way. And that is the key. Moving the air requires giving the air kinetic energy. How much? Well that depends on how fast you are going. Twice the car speed means the air has to move twice as fast. And kinetic energy increases with the square of the velocity. E= 1/2mv^2.
So, drag increases by the square of the velocity not because you are displacing more air or because you are displacing air at a faster rate, but rather because you have to impart exponentially more kinetic energy to the air to move it at the higher speed to cause the displacement.
At least, I think. Im not a physicist.
I guess I am the dog in this conversation.
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If you are intellectually inclined, read this:
https://physics.info/drag/Recall
Bernoulli's equation for the pressure in a fluid (air is a fluid)…Bernoulli nailed it….but I just think of it as an example of newton’s 3rd Law of Motion: There is an equal, but opposite reaction/force (from the air), exerted on the car, to the car’s force/displacement on the air.
“The portion of the drag force that is due to the inertia of the fluid — the resistance that it has to being pushed aside — is called the pressure drag (or form drag or profile drag). This is usually what someone is referring to when they talk about drag.
P1 + ρgy1 + ½ρv1^2 = P2 + ρgy2 + ½ρv2^2
“Drag increases with speed (v). I hope that this is self-evident. An object that is stationary with respect to the fluid will certainly not experience any drag force. Start moving and a resistive force will arise. Get moving faster and surely the resistive force will be greater. The hard part of this relationship lies in the detailed way speed affects drag. According to our sensible model derived from Bernoulli's sensible equation, drag should sensibly be proportional to the square of speed.R ∝ v2
In some situations, however, this may not be entirely correct. Drag is a complex phenomenon. It cannot always be described with equations that are simple.”
Last edited by CodyBigdog; May 25, 2022 at 08:32 PM.
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