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    • Thread Starter

    Hi there,
    I'm currently studying a level physics (OCR B) and doing my coursework for AS. I'm finding out the young's modulus of nylon and I want to find out the coefficient of friction of the pulley as one of my uncertainties.

    My modelling assumptions are that the system is in equilibrium and there is a light string.

    Help please!

    This is my diagram
    Attached Images

    How is it you plan on finding Youngs modulus from that problem? Are you given some elongation and stresses?

    With regard to your actual question, if the system is in equilibrium then why would there be any movement? if there is no movement then there is no friction, right?

    I might be wrong but im happy to spark off some discussion on the subject
    • Thread Starter

    (Original post by j55)
    How is it you plan on finding Youngs modulus from that problem? Are you given some elongation and stresses?

    With regard to your actual question, if the system is in equilibrium then why would there be any movement? if there is no movement then there is no friction, right?

    I might be wrong but im happy to spark off some discussion on the subject
    Yes you are right, I worded it quite badly. My experiment is to find young's modulus so i set up my apparatus as shown in my diagram. To calculate the YM I use stress/strain by measuring the diameter with a micrometer, measuring the length and extension with rulers and then calculating the weight of applied masses to the wire by using f=ma or f=mg in this instance. I believe I can use T2=T1euB where T2 is the tension of the pulling side,T1 Is the friction of the resisting side, e is the natural logarithm, B is the angle and u if the coefficient of friction of the pulley. I want to find the static or limiting friction.

    Oh i see, well some other factors to consider for your accuracy would be the compression of the nylon when putting the calipers on them (may or may not be applicable, depends on how you are doing it), the accuracy of the calipers themselves (found in manufacturers info), other things such as how you are applying the load and taking measurement simultaniously, there will be a critical point where your stress will reduce as the material further yields (stress relaxation) although your force is remaining constant.

    Anyway...the friction between the nylon and the pulley itself, its going to be extremely small (depending on what material the pulley is, presumably some polymer?) Suggestions to improve this would be using something like PTFE (teflon to you and me), also the bearing friction, if its too high then your nylon will have more friction against the surface of the pulley.

    To calculate the friction at the nylon-pulley interface, I would base it on rolling friction (but linear friction may occur like I said if the bearing in the pulley has high friction).
    See this link for more on rolling friction, http://www.engineeringtoolbox.com/ro...ce-d_1303.html it could be simple to calculate, but without a coefficient (see the link) you are going to get no-where. It's a good suggestion but i dont think you have to calculate it hope this helps.
    • Thread Starter

    Thanks for your reply. I looked at the link and I'm sure it will help.
    I've done the experiment in two ways, one with the pulley system and one using Searle's apparatus. I've got some data for both but these are my pilot runs, so i can get used to the experiments and reduce systematic errors caused by me and to identify others. Here are my uncertainties at the moment:
    • Micrometer +/- 0.05mm
    • extension/length +/- 0.5mm, with a ruler
    • measuring extension at right angle due to pulley.
    • calculations involving g, +/- 0.05ms^2
    • friction of pulley
    • clamp and pulley being in line which may cause the wire to not be taught.
    • bench i was working on was loftsided and wobbly.
    • Equipment was used by the school so there may be minor defects.
    • attaching tape to the wire to measure extension may not be accurate enough.
    • Yield point of wire

    And for Searle's apparatus:
    • The wire may not be taught
    • Keeping the meter to measure displacement level.
    • One of the scales on the meter was missing so i had to blue tack a ruler to it instead.
    • Slight friction on wrapping the wire round at the top.

    I thought the friction on the pulley, if it was great enough, may split the system into two parts if that makes sense, so the wire near the weight may have a different diameter. I will measure the diameter here next time.
    But if there are factors like I've listed where may affect the uncertainty, do you estimate them or just talk about them? As it's quite hard to estimate some of them.
    Searle's apparatus if probably going to give a more accurate result.
    I've made improvements to my method to prevent these uncertainties as much as possible but it's a lot to write lol.
    It's going to be hard to calculate the friction as I don't know any forces apart from the weight. I'm sure I'll get some marks for trying to work it out as it involves complex maths. I do further maths and I've finished M1 so it's quite nice to apply that knowledge. Although the OCR B course for Physics is annoying.

    Im not sure how your marking schemes work but I would certainly say you wouldnt have to calculate them all, if you can calculate then do so but you don't want to make this the focus of your report. You want to highlight the point, if the numerical result will have some benefit then include it, if it is just doing for the sake of it, then maybe it doesn't need to be in there?

    Another thing to mention is the human error, how well can you read off a ruler...+- 0.5 mm? it depends on the individual i suppose. Human error is always present. I do see what you mean though about 'splitting into two sections' assuming you mean the resistance in the pulley could cause the halfs of the system to become isolated from one another?
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Updated: January 19, 2015
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