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Measuring Tension Between Train Carriages

  1. Mar 2, 2015 #1
    Im doing a prac for school that we have to create ourselves.
    My groups prac goes along the line of a train pulling two carriages.
    Our train is a LEGO NXT unit that runs at a constant velocity, no acceleration. We will have two force gauges between each carriage and the carriage and the train and test how the tension changes as we increase the mass and run them up a incline. We want to compare the real world figures to how the tension would be as calculated on paper.
    Is there any equations that apply to no acceleration, constant velocity for calculating the tension?
  2. jcsd
  3. Mar 2, 2015 #2


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    If the train is going up the incline at constant velocity (no acceleration), then the tension between the carriages should be the same as if the train were stopped on the incline.

    Do you know how to analyze that situation?
  4. Mar 2, 2015 #3
    Sorry, im from Australia so im not quite sure what should be known and not known at my level in the US. Typically with this type of question, where given an acceleration in which we can then solve the net force and subsequently solve the tension force. Thats the way i would normally approach it. Not sure how i would approach it if ive got a constant velocity as the acceleration would be zero and thus the net force would be 0. How so would the tension force between them be the same as on the flat? They are acting a different angles. And in different trials different masses.
  5. Mar 2, 2015 #4


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  6. Mar 2, 2015 #5
    Though you have no linear acceleration, you have gravitational acceleration acting down the incline.
  7. Mar 6, 2015 #6
    Im aware of that. But how do i then put use that to calculate the tension force acting. Also, the prac was changed from two carriages to just one bigger one. I threw all the data, from the actual tension measurments from the prac in excel.
    And my dad (an engineer) gave me the following formula which im guessing is for some sort of rolling friction. (not we used frictionless surfaces).

    F = Weight (normal) x friction coefficient x diatmer of axle)/diamter of wheel
    x4 cause the trailer has 4 wheels.

    Can anyone point me towards some more infromation for this formula, in particular the derivation. It work quite well when i calculated the tension using and they were quite accurate and similar to the ones i actually recorded in the prac.
  8. Mar 7, 2015 #7
    Sorry for the delay in responding.
    I guess you are adding the rolling resistance force to the gravitational force then.
    (this is the tension force you need)
    If you assume that the weight of the carriage is evenly distributed around all 4 wheels, things are simpler.
    Get the rolling resistance force (f) in newtons ( fixed, regardless of speed ) from:
    f = m * g * ( cosine of incline angle ) * rolling resistance co-efficient
    ( m = carriage mass in kg, g = local gravitational acceleration in (m/s)/s )
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