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B Is the car accelerating?

  1. Nov 11, 2016 #1
    As I was driving I was wondering if there is way to know if the other cars on the road accelerating. I think that if I were travelling at a constant, then if the rate at which the distance my car and another car was changing was not constant, then the other car would be accelerating. But is there any easier way?
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  3. Nov 11, 2016 #2
    Match their motion with your motion.
  4. Nov 11, 2016 #3


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    Using the above procedure, if a car is driving circles around you, [you would decide] it is not accelerating.

    Edit: clarification of intended meaning inserted
    Last edited: Nov 11, 2016
  5. Nov 11, 2016 #4
    You could try to keep the distance constant and see if you are accelerating or not.
  6. Nov 11, 2016 #5
    Can you just look at them move? If they are turning, or speeding up, or slowing down they are accelerating.

    You can look at the way the drivers lean. People lean in the direction of acceleration to stay balanced. If they have one of those air fresheners or toys hanging from their rear-view mirror, you can look at the way that moves. It will swing away from the direction of acceleration.

    If you keep the same distance and heading to them, then you can feel if you are accelerating by the fictitious forces you feel.
  7. Nov 11, 2016 #6


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    not sure if you were pointing out an error in the OP's understanding ?

    Because of course, an object with a circular motion is undergoing acceleration ... the car doing the circles around you
  8. Nov 11, 2016 #7


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    I wondered the same thing. Upon reflection, I conclude that, yes, he is pointing out a flaw in the OP's method.

    A car going in circles around you, is neither moving away nor toward you, thus the OP would (erroneously) conclude it is not accelerating.

    It's a bit contrived though. He did say "other cars on the road". Doing circles around you would be quite an edge case.
    Last edited: Nov 11, 2016
  9. Nov 12, 2016 #8
    Yeah that is a good point. I know any car changing direction would be accelerating. But I am wanting to know how to determine if a car travelling in a straight line in the lane beside me is accelerating. For example, if a car in the lane beside me which was initially in my blind spot rushes ahead of my car while remaining in its lane, how would I know whether the car was travelling at a very high speed from the beginning or it suddenly accelerated?
  10. Nov 12, 2016 #9


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    Now that is a very different question.

    (In practical terms - if this hypothetical scenario involves a real life moving violation - it doesn't matter which. You are obliged to be sure the lane is clear - They have right-of-way. If you pulled in front of another car, you did not make sure the lane was clear. But I digress.)
  11. Nov 12, 2016 #10
    I am not saying they're passing me. The car would remain in the lane beside me as it sped forward.

    My question remains, how would I determine if the car was accelerating or not?
  12. Nov 12, 2016 #11
    Well your in your car, see if the speedo changes it's on the dashboard.
  13. Nov 12, 2016 #12


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    These two statements directly contradict each other.
  14. Nov 12, 2016 #13


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    but you did state they were passing you

    I don't think you can, unless you observed earlier/prior motion of the other car

    velocity is a vector comprised of speed and direction
    Acceleration can be defined as a change in either of those

    take my earlier comment .... an object ( car or whatever or a planet around the sun) travelling in a circular motion is accelerating, even if the speed is the same the velocity isn't because the direction is changing

    the car in the lane beside you may not be changing direction, but if the speed is changing, then the velocity is changing and therefore there is an acceleration

  15. Nov 12, 2016 #14
    If you were moving at highway speed and the other car "sped forward", or sped by you, I would say it is likely that he had a fairly significant speed delta relative to you initially. There are exceptions, but at freeway speeds most cars are not able to accelerate very quickly due to various factors (wind resistance, gearing ratio). If he was sitting in your blind spot matching your speed for even a short time, and his car had a lot of power, he could have accelerated relatively quickly. But even so, the distance from your blind spot to a position where you could see him out your window is a relatively short distance. So even a fairly powerful car could not accelerate a great deal in that short of a distance. If he was accelerating hard, were you able to hear his engine roaring? Then again, I suppose the reason you are asking is that this was some kind of a traffic incident, in which case you (or the person involved) probably didn't hear him.

    Obviously, if it was at lower speeds on city streets, the other car could accelerate more quickly. But even if he did accelerate quickly, if this is a "who's at fault" kind of issue, I'm sorry to say it, but I don't think you have a good case if you changed lanes and hit him. Then again, I'm just speculating as to why you are asking about this.

    If you were shown a very short snapshot in time (a very small fraction of a second) of another car relative to your car, you could probably estimate its distance from you fairly accurately. But if you were asked to estimate acceleration, or even speed, you would need to observe it for a longer time to be able to observe change over time. When I had the opportunity to use a radar gun, it had an audible tone that increased or decreased pitch as the car's speed increased or decreased.
  16. Nov 12, 2016 #15
    Ok. Just to clarify there was no traffic incident, this was just question that popped in my mind. I was just wondering if there a visual way to determine if an object is accelerating when you yourself are in moving reference frame. I guess there is not.
  17. Nov 12, 2016 #16


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    There is a fairly straightforward way (easy to describe, at least, but not so convenient to implement). You need to be able to measure the bearing, velocity and range of the other car over time and also the bearing, range and velocity of a point on the road over time (both in your reference frame). This will yield what the other car is doing in the Earth's frame. Not easy for the car driver but achievable by a passenger with a hand held speed-check device (LASCAR, for instance) and a hand bearing compass. If you can't measure range directly (LASCAR forbidden) then you would need a parallax method (as in astronomy) using the change of angle subtended between local and distant points and the speed over the ground and heading of your car (heavy!).
    This is a similar problem to that which was encountered with early Marine Radar systems which, originally, just showed bearing and range of another ship. With no other information than what you saw on the Radar display, both ships could spiral in towards each other, whilst faithfully following the 'pass Port to Port' rule. (see Radar Assisted Collision). But they didn't know where they actually were; in your car, you (hopefully) do.
  18. Nov 12, 2016 #17


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    of course there is.... do some visual and timing measurements
    measure the time taken for your car and the other car to move between successive sets of road markers or power poles.
    compare their time with your time.
    If their time is decreasing or increasing between sets, they have positive or negative acceleration.
    If their time doesn't change, they have a steady velocity

    the same for your car
  19. Nov 13, 2016 #18


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    Presumably this is just one of the issues that driverless car systems have to handle.
  20. Nov 13, 2016 #19


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    The use accelerometers and radar.
  21. Nov 15, 2016 #20
    when the car next to you is matching your speed, you can be braking or accelerating and you will know if its accelerating if you are accelerating. (your spedo is going up)... without a speedo, you need to monitor the rate of change of the things around you . after all, acceleration is the rate of change of speed.
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