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Earlier today, I was talking with my friend about speed cameras measuring different speeds when moving.

This is the situation:

  • I am the red car and I'm driving 61km/h.

  • The green speed camera is getting pulled 50km/h by the blue car.

  • The speed camera is aimed at the left and measures my speed as I pass it.

drawing

My friend said that the camera would still measure 61km/h. I say the camera measures more (maybe 111km/h?) since how I understand, in relation to the camera, the red car moves faster than it moves in relation to the ground.

He couldn't give me an explanation of his theory, but he is sure.

Am I right or is my friend right? If he is, why is that?

Thank you for your help from us both.

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  • $\begingroup$ It would depend on the specific system. It seems like radar which can account for the measuring car's speed is fairly common. $\endgroup$ – JMac Oct 17 '19 at 13:55
  • $\begingroup$ Re, "He couldn't give an explanation...but he is sure." It's a sad fact, but you probably will have a hard time winning that argument. If your friend is not interested in talking about what speed means or, in knowing how the camera measures it, then the argument is never going to rise above the level of, "Is not! ... Is too! ... etc." $\endgroup$ – Solomon Slow Oct 17 '19 at 14:13
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    $\begingroup$ You are both wrong. With the camera aimed to the left as shown it will measure the speed as 0 in both cases. $\endgroup$ – alephzero Oct 17 '19 at 14:14
  • $\begingroup$ @alephzero why is that? And what are the two cases? $\endgroup$ – gwynbleidd Oct 17 '19 at 14:20
  • $\begingroup$ What do you mean by "speed camera"? If you mean radar, then the answer is 0 as stated by @alephzero. A radar gun only measures the speed parallel to the direction that it is fired - which in this case is 0 because you're using the gun perpendicular to the direction of motion. If you mean something other than radar, then specify how your device works. That detail matters. $\endgroup$ – Brick Oct 17 '19 at 19:40
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Simply, it depends. There are also some speed cameras which detect the speed of vehicles irrespective of whether they are themselves stationary or in motion. So, it depends on what type of camera you are using.

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The camera will not measure 61 km/h, as speed cameras are meant to be used from a stationary frame. Speed cameras measure speed by measuring the rate of change of the angle between the camera and the car, and measuring the movement in the image with respect to other parts of the frame. All of these things mean that the speed camera will not give a reading of 61 km/h if the camera itself is on a moving vehicle.

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