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B How to calculate the deceleration of the object

  1. Sep 4, 2018 #1
    Hi, i am trying to understand, how to calculate the deceleration of the object?
    I have a small toy car in the track, after i release the car, i wanna calculate the deceleration of the car, and predict how many loops, it's going to do, after measuring the timing of some revolutions. Is there any deceleration formula or something like that?

    Ps. Sorry for my english, i am not from USA.
     
  2. jcsd
  3. Sep 4, 2018 #2

    Nugatory

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    You need to know what forces are acting on it to decelerate it. In this case it will be friction and air resistance
     
  4. Sep 4, 2018 #3
  5. Sep 4, 2018 #4

    CWatters

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    If you can measure the speed of the car at two points and the time it takes to go between them you can calculate the deceleration.
     
  6. Sep 4, 2018 #5
    OK, i will try to do it. I get one problem, the deceleration, has to increase every time, or once i run the object, it's constants?
    Let's say, for example, the deceleration is 1cm/s, it has to increase, or from start to the end, it will be 1cm/s?
     
  7. Sep 5, 2018 #6

    CWatters

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    That depends on what causes the deceleration. If it's air resistance then the drag force isn't constant so the deceleration isn't constant.

    If it's friction in wheel bearings it might be constant.
     
  8. Sep 5, 2018 #7
    You mention the car doing “loops” which I assume means the car is going around a track. If the track has straight sections and curved sections the resistance will certainly be different in those regions.
     
  9. Sep 5, 2018 #8

    BvU

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    Is this homework ?
     
  10. Sep 5, 2018 #9
    I'm new here and not sure what level you are coming to this from. I assume it's some kind of wind-up toy car or there is some way to change the release speed. If it’s a wind-up, it may accelerate after release before it starts slowing again. Aero drag is D= ½ p V^2 S Cd so D force will be greatest at the highest speed. Rolling resistance is a constant though.
    D = Drag Force
    p = density of fluid (from memory) 1.225 kg/m^3 air at sea level.
    S = Frontal surface area
    Cd = coefficient of drag


    Sounds like there could be a lot of things to account for and many unknowns. It might be simpler to do the coast down tests and record results in Excel, you can then use excels regression tools to produce a mathematical model for the deceleration in relation to starting energy/speed etc. The accuracy of results will depend on the regression curve type you use. Try different curves and pick the one that gives the best correlation coefficient. It won’t be physics formulae but it will be a practical predictive model.


    If you don’t understand what I am saying, I am aiming my answer at too high a level (not sure how that works yet) so apologies.
     
  11. Sep 10, 2018 #10
    I get the timings, but if i try to calculate the deceleration, everything looks so random... Something is not working.

    All i do, is: endingVelocity - begingVelocity / << What time put here? Should be 1000ms, or the last endingVelocity, from the last revolution?
     
  12. Sep 11, 2018 #11

    CWatters

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    Can you describe the motion of the car. Is it going round a loop (vertical circle)?

    Can you make a photo of the track?
     
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