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Homework Help: Experiment on pendulums

  1. Aug 18, 2004 #1
    I need to design an experiment and I don't quite understand it, the aim is to find out the factors that affect the period of a simple pendulum.
    The questions that were provided are:
    - Define a period in a simple pendulum motion.
    - List possible factors that could affect the period.
    - Write a suitable method that can test one factor at a time.
    - What are the variables?

    Hopefully you can help me!
  2. jcsd
  3. Aug 18, 2004 #2
    Im sure someone else can do a better job, but i'll try to answer some of your questions. First of all, if i remember correctly, one period of a pendulum is the time it takes for a complete swing to happen (by complete swing, i mean the weight swings from one side to the other, and back). The factors that affect the period are, the length of the string, the mass of the weight, and the angle at which it is released (how far you pull the weight back). As for testing...you just need a stop watch, protractor, some weights, and some string. Do a bunch of trials keeping all the variable constant, except ONE. First measure the period using different length strings but same mass and angle, then with different angle but same length and mass, then different mass but same length and angle. The variables are the angle of release, mass of weight, and length of string.
    Last edited: Aug 18, 2004
  4. Aug 22, 2004 #3
    thankyou for the information
  5. Aug 22, 2004 #4


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    Just an advice when taking the period:
    Let the pendulum swing back and forth several times (that is, several periods, say 5-10) before you stop your watch.
    The period is then gained by dividing by the number of full swings you let the pendulum take.

    You can figure out for yourself why this is a good idea
    (Hint: Assume you are not able to stop your watch exactly when the pendulum reaches its topmost position..)
  6. Aug 23, 2004 #5
    Thanks for the advice arildno,
    but I'm still a little confused: What is the definition of a period in a simple pendulum motion?
    from the information I've found and what you have told me, I think it is the time taken for the weight to swing from one end to the other and back? That is considered as ONE period right?
    But with the experiment, I am provided with a pendulum and the other equipment I have to figure out what I'm going to use and why. I know I need a stop watch to time the period, but as you said it will be difficult to measure the period due to the reaction time!
    I'm confused. Please help!
  7. Aug 23, 2004 #6
    This is correct, one period is defined as the time it takes for the pendulum bob to return to it's original position.

    As mentioned before, you will need a few different size lengths of string, a protractor, different masses and a stop watch.
    The longer your strings are, the easier it will be for you to take those measurements (this should come through intuition).

    When measuring the period, by starting your stopwatch a couple of periods after you release the bob, you will give the pendulum time to 'settle' a little bit (from slight fluctuations your hand may have given it). By stopping the stop watch after a number of periods (and then dividing to find the actual period) you will be reducing the uncertainty in your measurements.

    Another thing you might want to consider----- think about what force is making the pendulum move? Will it move as fast on the moon?

    Basic outline of an experiment might be:

    Take several measurements keeping the mass and angle of release constant, but changing the string length each trial.

    Take several measurements keeping the string length and angle of release constant but changing the mass each trial.

    Take several measurements keeping the string length and mass constant, but changing the angle of release each time.

    What is the effect of changing these things? (Does the string length change the period? Does the angle of release change the period? Does the mass of the bob change the period?)

    See attached file for a basic set up

    Hope this helps
  8. Aug 23, 2004 #7
    Thanks a lot tyco05!!
    I think I'm getting a better understanding of what I have to do and how to set certain things up, thankyou very much everyone, I really appreciate it! If I need more help, I'll get back to guys, THANKS!!!
  9. Aug 24, 2004 #8
    ok, I'm stuck AGAIN! This is really bad!
    I'm sure I know how to use the different length strings and different masses, but what equipment do I use so I can hang the string and masses on? Also, how do i measure the angle at which the mass is released?
    So, if the mass was hanging down, it would be perpendicular from where it is hanging from, and so that would be 0 degrees, and say if i moved it up 90 degrees, how do i know or measure that 90 degrees?

    One last thing, when i let go of the mass, i start timing, and let it swing for say 10 periods and stop the watch. Divide the time by 10, that's 1 period? and repeat...etc?

    Ok, hope you guys can help again!!
  10. Aug 24, 2004 #9
    For a crude set up you could use a retort stand with one of those clamp things on it (can't remember the name) or just a straight metal rod and hang it over the edge of a bench. See attached. You can tie the string at point * , and be sure to attach the protractor (maybe by blue-tac or something) to the end of the rod. (use the first picture I attached a few posts ago as a front on view). Be sure to measure the length of string to the centre of mass of the bob. (this can be approximate if you like)

    When the string is straight down, take that as 0 degrees, yes. I suggest only taking measurements up to 45 degrees, maybe in 5 degree increments. (ie take measurements at 5,10,15,20,25,30,35,40,45 degrees.
    Past 45 degrees can make things a little trickier come analysis time.
    Yes. That's 1 period.

    If you do a little research (I'm not going to tell you straight out!) you may find a formula or two that you can check and compare your results with. Then you will be able to discuss reasons why your results differ (which they no doubt will) from the results obtained from said published formulae.
  11. Aug 24, 2004 #10
    Are my attached files working??
  12. Aug 25, 2004 #11
    Thanks tyco05!!! really appreciate your help!!
    makes things very clear, although I can't find the attached file you are talking about?
  13. Aug 25, 2004 #12
    G'day again,

    I think I have attached the files to this message (hopefully they work).
    If you need any other help just let us know mate,


    Attached Files:

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