1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Lab Experiment - Uniformly Accelerated Linear Motion

  1. Sep 21, 2009 #1
    Hi there, I'm not sure if I'm posting in the right place or not... So please forgive if I am.

    I did an experiment today on Uniformly Accelerated Linear Motion. We used an Atwood's type machine that used pressurized air to reduce friction. We also used photogates to time the movement of the horizontal weight as accurately as possible.

    I have done all my calculations but am coming up with some figures for gravity that are too low. No biggie... Our physics professor says to always just explain what you believe skewed the results... Again, no biggie...

    So I decided to calculate the initial velocities that would have had to be in place for the results that I ended up with. Again not a problem and I come up with negative velocity values across the board (10 trials of equivalent weights but differing distances was done).

    So I am thinking that either friction from the track, OR some sort of drag caused by the air resistance from the pressure/velocity of the air coming out of the holes in the device (therefore vertical) is the culprit.

    Here is my problem. I don't know how to calculate either one yet to speculate on which is the most likely cause.

    This question is sort of related to homework, but I am not seeking any help other than direction on how to calculate the "negative force/resistance" caused by the vertical blowing air on a horizontally moving object. Or would that case be impossible since there is also air coming out from holes behind it to basically propel and zero that out? I do suspect that this is the case.

    If so, then how would I calculate any friction caused by there possibly not being enough air flow? I know I would need to take gravity, the pressure of the air, and the materials of the device (aluminum) into account... but after that I'm at a loss.

    I'm sure my professor would be satisfied enough that I calculated the necessary initial velocities to be in place for my results, and what I thought (basically a conclusion that amounts to a lot of what I've written here)... but just for the sake of being extra thorough, I am curious...


    I'm posting this part after working with this a little more:

    Okay, I have derived the negative acceleration based on my negative initial velocities and the times... and I come up with figures that are VERY close to that of known gravity (and almost identical across every trial)...

    So am I correct in thinking that I can use this as justification to conclude that what skewed the results HAD TO BE friction? And couldn't I turn that into a Newton's force by multiplying that by the weights used (converted to kilograms of course) and call it the "opposing frictional force" without getting into all the rest of the complicated air pressure and material constants that the friction formulas I find seem to require?

    Thank you guys for any advice...
    Last edited: Sep 21, 2009
  2. jcsd
  3. Sep 21, 2009 #2


    User Avatar

    Staff: Mentor

    If the air table is horizontal, there is very little friction or air retardation. Was this table tilted? If it's tilted, then I think you will get different air flows in front and behind a disc...?

    Welcome to the PF, BTW!
  4. Sep 21, 2009 #3
    Thank you so much,

    I thought my partners tested it with the level we had, but they may have just looked at it too quick and it was actually out of level.

    That would explain how I am coming up with an opposite constant acceleration, and one that is SO close to gravity... The device wasn't level! I don't know why that didn't dawn on me with it staring me right in the face! Must be time for a break...

    Anyway, thank you again and thank you for the welcome!

    This forum is great!
  5. Sep 21, 2009 #4
    Okay, I'm back... break time is over...

    I'm so excited! And in looking at this further, I can actually average the gravities that I came up with, and the opposing constant accelerations... coming up with 2 vectors right?

    Then if I set these vectors at 90 degrees from each other (1 in 1st quadrant, and 1 in 4th -- both at 45 degrees from the x-axis) I can then "add" (combine) them into 1 resultant vector. This new vector should end up in the 4th quadrant with a theta degree to the x-axis that is very small....

    ... BUT that will also be a fairly accurate degree that the device was out of level... Right?
    Last edited: Sep 21, 2009
  6. Sep 22, 2009 #5
    Whoops, that actually would not be the right way to calculate that angle of difference that it is out of level.

    I would have to know the length of the machine and use it as a "radius"... then calculate it that way, wouldn't I?

    And of course I have no measurement for the length of the entire machine, so that much will just have to stay out of the report.... <Sigh>
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook