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Pulley tension and friction

  1. Oct 27, 2012 #1
    1. The problem statement, all variables and given/known data

    So i did a lab in school this week and i have to write a report but i am having some problems getting friction. so i have two masses a 1703 gram block and a hanger that pulls the block that has 700 grams on it. The block went a distance of 45cm in a time of .41 seconds. i have to get the tension and friction of it

    2. Relevant equations
    i used S=1/2(a)t^2
    i also know i am supposed to use T-fk=m1*a and t-m2*g=m2*a i think



    3. The attempt at a solution
    i used the first formula for the acceleration and i got 5.354 m/s^2and after this i tried finding the tension in the string and i got 3.1122 N which i am not completely sure is correct but the main problem is i cannot find the friction i tried but i keep getting answers over 1 or negative answers and i am stuck. i might have done the lab wrong and gotten wrong weights or something but i am not sure.
     
  2. jcsd
  3. Oct 27, 2012 #2

    TSny

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    Hi wuki1. Welcome to PF.

    Your equation t - m2g = m2a looks like it might have some problems with signs (unless you are taking the acceleration to be a negative quantity for this equation). But, I do agree that your data gives a tension of 3.11 N and a force of friction that would need to act in the wrong direction!:confused:

    To see if your acceleration value of 5.354 m/s2 is reasonable, you might do a calculation in which you work out what the acceleration would be if there were no friction in the system (using just the given masses and Newton's 2nd law).
     
  4. Oct 27, 2012 #3
    finding it using masses i would do (m2/(m1+m2))*g right? if so i got 2.85 m/s^2 and when i than plugged this acceleration in to get a different tension and friction it did work, so should i just take it as me doing something wrong and getting a wrong result for the time which gave me a bad acceleration?
     
  5. Oct 27, 2012 #4

    TSny

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    That looks right. So, without friction the acceleration would be 2.85 m/s2. So, with friction, it would have to be less than that. Therefore, there must be something wrong with your data. But, it's hard to say which of your measurements is (are) wrong. If you know your values for the masses are correct, then there must be an error in the distance or time measurement. If I had to guess, I would say that the error was in the time.

    Just to make sure I'm seeing the setup correctly, does the attachment essentially represent your setup?
     

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  6. Oct 27, 2012 #5
    yes that is the correct setup and yeah the masses and distance are definitely correct so it must be the time that is wrong, it surprises me that the time could make such a huge difference.
     
  7. Oct 27, 2012 #6

    TSny

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    Well, the time is squared in your calculation for the acceleration. It seems to me that 0.41 s is a short time for the block to move .45 m. When you did the experiment, did it seem like the block was moving for less than half a second?
     
  8. Oct 27, 2012 #7
    it did go pretty fast when i did the experiment the problem was i wasn't the one measuring the time so i cant be completely sure if it was correct but yeah .41 seconds might have been too fast
     
  9. Oct 27, 2012 #8

    TSny

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    If you were using photogates to measure the time, then you would think it would be fairly accurate. But if someone was using a stopwatch, then there could be significant error in measuring such a small time due to reaction time in starting and stopping the watch.
     
  10. Oct 27, 2012 #9
    We were using stopwatches in this experiment so that is what is wrong with my data. Thank you for helping me figure this out.
     
  11. Oct 27, 2012 #10

    TSny

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    OK. So, you can see that it would be good to make many time measurements by different people in your group and average the results. Of course, that's not of much help now. :redface:
     
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