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Homework Help: Rolling friction and work done

  1. Dec 22, 2009 #1
    1. The problem statement, all variables and given/known data
    I preformed a experiment to find the joules of work taken for my elastic band car to travel a distance of 18.1 meters

    Does my result seem correct?
    Last edited: Dec 22, 2009
  2. jcsd
  3. Dec 22, 2009 #2
    Your pic isn't displaying.
  4. Dec 22, 2009 #3
    noticed that the second i posted, fixed now
  5. Dec 22, 2009 #4
    This is my method

    To find the coefficient of friction for a rubber band car.

    • Rubber band car
    • Ramp
    • 2 rulers
    • Measuring tape
    • 2 Digital Cameras capable of 30 fps

    Experimental procedure:

    1. Create a ramp and where it hits the ground mark off 10 cm distance straight ahead.
    2. At the end of the 10 cm mark, mark 1 meter then another 10 cm.

    3. Set the cameras up so each one records a 10 cm marked segment at 30 fps
    4. Release the car from various heights (adjust the ramp)
    5. Analyze the video to determine the average speed of the car at the beginning of the horizontal motion and at the end of the horizontal motion.
    6. Record data in chart as below for each run.


    1. Using the equation V2^2 = V1^2 + 2ad; solve for the acceleration of the car.
    2. Since F = ma = umg, the mass is not a factor and u = a/g.
  6. Dec 22, 2009 #5
    This looks pretty reasonable--the math is sound,what are the rubberbands for?
  7. Dec 22, 2009 #6
    you mean the strength? they are staples #33 , the small normal ones or maybe slightly thinner.
    I used a light flywheel design car that went 18.1 meters but moved slowly, the rubber band was wrapped around the axle
  8. Dec 22, 2009 #7
    what worried me was the varying values for the coefficient of friction, this was all with the same car and that value shouldn't change, I couldn't find where i went wrong in the math and repeated tests came out at close to the same exact values
  9. Dec 22, 2009 #8
    No i wondered whether the rubber bands contribute to propulsion. If so it may explain some of the variation in the data. The rubber band would need to be slack at both measurement intervals, otherwise you are complicating this problem needlessly.
  10. Dec 22, 2009 #9
    oh, yes of course for the 18.1 m run it used elastic propulsion but for the lab trials it used a ramp(and the elastic unattached)
  11. Dec 22, 2009 #10
    Ok good. Hey welcome to real science. If you have more than 20% variation in the data, that would be odd. I help with the pinewood derby races at the scouts--basically what you are talking about. The results can be very close and astonishingly repeatable on a good track--so lets revise that number to 5% or less.I think you have rounded your calcs off but otherwise seem reasonable--two at 0.01, and one at .00 (which can't be!).
  12. Dec 22, 2009 #11
    yes i used excel to round
  13. Dec 22, 2009 #12
    which means that although the values show rounded they are calculated to many many decimal points, so only the results are rounded, no rounding during the calculations
  14. Dec 22, 2009 #13
    thank you for the reassurance and guidance
  15. Dec 22, 2009 #14


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    Homework Helper

    Can you collect more data for different ramp heights? It seems that the higher the ramp, the greater the deceleration. This may be due to air resistance or other forms of friction which are greater for higher speeds.

    Also, when analyzing the video, did you count the number of frames from the time the front of the car entered to the time that it left? If you count the total number of frames the car was visible, you have to take into account the car's length.
  16. Dec 22, 2009 #15
    i made the mistake with the length of the car the first time but i deleted that data, my car is broken but ill try to get it fixed for after christmas
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