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OldTimeyMedicine t1_j1ep6j9 wrote

I think it's Car Speed, it only seems like the ground is moving but it's actually the car

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Schroedinbug t1_j1f1tjp wrote

More like wheel speed. Not ground speed due to things like drifting or dyno, not air speed due to wind, not car speed as you can lock the brakes up and it'll show zero while sliding into the sunset at speed. GPS speed would be closer to "car speed", but there are still issues with z-axis movement.

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hearnia_2k t1_j1ewg7i wrote

Wouldn't it be both? Equal and opposite forces blah blah means if a car drives in one driection then it applies the same force on the ground in opposite direction. So, the ground speed is the different between the speed of the ground in one direction and the car in another direction.

Otherwise the car would have to do some pretty unnecessary calculations to know how much force the car was putting on the earth in the opposite direction, and how much that impacted the speed of the earths rotation etc. Plus the speedo would likely have to show pretty close to 1.3 million MPH?

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OldTimeyMedicine t1_j1ezp6r wrote

Setting aside speedos for a moment, the Car Speed would be relative to the frame of reference, or more appropriately, the rate at which the wheels are turning since the speedometer is measuring that based on the wheel rotation. So if the car is drifting, the speedometer will present the speed according to wheel speed rather than the resolved components of the speed vectors, where the datum isn't defined here.

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hearnia_2k t1_j1ffmaf wrote

Yes, it's true, and I even mentioned this in another comment, but it's also not what you previously states, where you said it's the speed the car is actually moving; but it's not, it's based on having a value for the outer circumference of the tyre, and speed of rotation etc.

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hackulator t1_j1ggoln wrote

Well, all motion is relative so it really depends on your point of view.

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