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Friction of objects sliding down a ramp -- does mass matter?

  1. Oct 1, 2015 #1
    I have been teaching Physics at the introductory level now for over 30 years. In that time I have taught a lot of labs on friction. Using the small masses readily available in the lab and using motion detectors to measure the velocity of the objects neither I nor any of my students has ever measured heavier objects sliding down the ramp at the same speed as lighter objects. I know what the theory states. I am talking about laboratory observations.

    I also know that air drag can be significant for bob sleds, rolling cart, bicycles, etc., but I am talking about objects moving at very small speeds (significantly less than 1 m/s) so I don't believe that air resistance matters here. I have looked on line to see if there are any demonstrations that might give a clue as how to show what the theory says should happen, i.e. what conditions need to be met, but so far I have found none. I have seen demos where the two masses clearly arrive at different times (small difference but entirely noticeable) and heard the presenter say see no difference.

    So if anyone here has some real insight as what might be causing the lab masses to act differently than theory predicts i.e. the larger masses slide down faster than the lighter ones, I would welcome the insight.
  2. jcsd
  3. Oct 1, 2015 #2


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    Well, which theory? Most physics courses use Coulomb's theory of friction, which apparently also subsumes a couple of earlier theories:


    Friction is still a pretty complex phenomenon to analyze.

    In trying to analyze what is going on in a video demonstration, you are never certain of the conditions under which the demonstration is taking place.

    As a physics teacher, why don't you try to answer your own question? This seems like something which could be used to develop an experiment to gather data for analysis.
  4. Oct 1, 2015 #3
    Well like I said I and my students have been doing the labs for a long time. And what we see is that larger masses slide down faster. Does that agree with any theory you know of Coulomb or otherwise. Keep in mind that I'm talking about very low speeds.

    I assumed that since I said I had been doing labs for more than 30 years that it was obvious I had been trying to figure it out. Your reply is rather insulting. If you don't have anything to contribute yourself then why are are answering. Just trying to put me down?
  5. Oct 1, 2015 #4


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    The thing about anonymous forums is that you don't always get a complete history of what someone has done in only a few lines of text, just like you can't always depend on what you see in a video pulled out of the internet's hat.

    I think the article I linked to had a rather extensive list of references, which should indicate that friction is complex to analyze, that theory is still probably outstripped by empirically derived relationships.

    We're not trying to ring anyone's bell here at PF. Why are you so defensive when someone makes a suggestion?
  6. Oct 1, 2015 #5
    I suppose I reacted a bit hastily. The reference you listed gives a the standard Coulomb theory for dry friction. What I would like to find is either some corroboration of what I and my students see with some kind of explanation or something that defines parameters needed for the Coulomb relation to hold.

    I have only empirical data, I teach between 15 and 21 semester hours per semester. So not much time to carry on my own detailed experiments. But every year when we do the friction lab and the results don't align with what the theory in the book says they should, I'm a bit frustrated. I am also tired of the standard excuses that: we must not be meeting the assumptions made by the theory. I have not found any concise statements about the assumptions necessary for the Coulomb relation to work. It's supposed to work for dry friction.

    It would one thing if there were a lot of variation, with different masses falling faster sometimes, but what we see is that every time the heavier masses fall faster.

    So if anyone has anything that explains conditions where we would expect to see heavier objects falling faster than lighter objects I would like to hear it.

    If there is a regime where small metal cylinders sliding down a wooden ramp exhibit a viscous like friction I would like to read about it. I have looked on the internet and not found anything that is satisfactory. Our library has very limited physics holdings.
  7. Oct 1, 2015 #6
    Is it possible your heavier weights have polished their sliding surfaces more than the lighter ones?
  8. Oct 1, 2015 #7
    That is a very good question. I use some standard weight sets that have been around for a long time. I have also placed different amounts of weight on a short piece of a 2" x 4" pine. I have tried covering the ramp with white postal wrapping paper. It might be worth it to invest in some small plastic sleds and see if that changes the results.

    I will try it out and report back.
  9. Oct 1, 2015 #8


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    In a series of 1974 videos about gyroscopes, there's one with a video of flat block like masses of different on a somewhat slippery surface like a white board. The board is tilted until the blocks slide, and the smallest ones slide last, so at least the dry static friction coefficient is greater for the smaller masses, so it would seem to relate to the tests you ran. The demonstration is in video #2 of this backed up web site:


    I'm not sure why this happens.
  10. Oct 1, 2015 #9
    Thank you. That is a very interesting video. I have no idea why it happens either but that is the behavior I have been seeing in my friction labs for years.
  11. Oct 1, 2015 #10

    Mister T

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    The prediction that light and heavy objects slide down an incline at the same speed or acceleration is based on the simplifying assumption that the friction force is proportional to the normal force. Or in other words that the coefficient of friction is constant. That simplification is only an approximation.

    As someone who has also taught introductory physics for about 30 years, my professional opinion is that far too much is made of this silly relationship between the friction force and the normal force. Yes, I teach it. But I don't find it productive to dwell on it for extended amounts of time. We do a simple lab activity where the friction and normal forces are measured for a block sliding on a table top. The mass of the block is changed, and the forces measured again. Do this a few times and make a graph of the friction force versus the normal force. Draw a best fit straight line and equate the slope to the coefficient of friction. You can see for yourself that the graph is not really a straight line.

    Did you know that accident investigators use a velocity-dependent coefficient of friction?

    The friction force and normal force are not really separate forces. They are perpendicular components of the contact force. But I digress ...
  12. Oct 1, 2015 #11


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    Can you provide any details/data about the experiments? You haven't provided much to go on. How much mass? What angle? What type of surface? That are the masses made of? What are their dimensions?
  13. Oct 2, 2015 #12
    It's pretty simple, according to the lab manual I inherited, the students are to try to measure the coefficient of friction of a wood block first on a horizontal table top covered with paper. They drag the block across the table with about 5 different mass loads added: the mass loads range from a 100 grams up to a kilogram. Their results for the coefficients of static and kinetic friction look reasonable and they take the average of the 5 different mass load trials. Then they are supposed to do a dynamic measurement by sliding the wood block down an incline with 500 grams and then 1 kg. The slope of the incline is just enough that the masses slide without stopping and starting around 20 to 30 degrees. The results of the block sliding down the incline do not agree with the previous results at all. And the heavier mass loads almost always start moving at a lower angle and they have a measurably larger acceleration than the lower mass loads. Upon seeing this behavior I have looked at just sliding some of the masses in the store room down the incline and if the masses are reasonably smooth the heavier masses always seem to have a larger acceleration.

    But my question is not really about my specific labs it more about the validity of the Coulomb model of Friction. And from a few of the posts and a few websites that I was led to I believe I now have the answer that I was looking for. Namely the Coulomb model is not all that good. As busy as I am with my teaching load I have simply never looked hard enough or in the right places until now. And I still have a bunch of papers to grade and assignments to update and tests to get ready for next week. So for now I am quite satisfied with the answers I have received here.

    Thank you all very much.
  14. Oct 2, 2015 #13


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    Thread closed at OP's request.
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