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Thread: average force of friction

  1. #1 average force of friction 
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    hi guys,
    i am having trouble with this problem.
    Calculate the average force of friction acting on a wood block as it is propelled along the floor by an elastic band ?

    my solution and my ideas:
    What does it mean by average froce of friction?
    I first weighed the peice of wood to find its mass. i then used a newton scale and pulled the elastic band 60 cm and found the force of friction which was 8n. is this force applied(8n)?did i measure the force correctly of the elastic.?i have no idea where to go from here. is there any kinetic energy involved? do i use Ek=1/2 mv^2. any suggestions would be helpful. do i time the wodeen block after i let it go and find the velocity. i am sure it is uniform acceleration.
    thank you.


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  3. #2  
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    I agree with you this question is confusing. By average they could mean that there is some variation of the coefficient of friction as you pull it along the floor. But then how would you measure the average. I guess you would read the scale and kind of eyeball where you think the scale mostly was reading as you dragged it along. I don't know what the elastic band would have to do with the price of tea in China. Then again maybe the elastic band will stretch a little bit until you overcome the static friction, after which it will move under kinetic friction resulting in a jerky kind of motion. But I don't know what an average of the static and kinetic friction would tell you. The energy is only a function of kinetic friction, because no work is done as the block is standing still. I'm afraid I probably didn't help you much.


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  4. #3  
    M
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    Typically we consider two types of friction, static friction (the force you need to overcome to get the resting block into motion) and dynamic friction (active as soon as the block is moving). I believe you are asked to obtain the coefficient of dynamic friction.

    Theroretically, dynamic friction is independent of velocity, it's simply a constant friction coefficient times the weight of the block (moving on a horizontal surface). Why "average"? a) the theory is not exact (as the underlying assumptions are not exactly valid), b) you're doing this by experiments and that's not exact science, either. However, although speed does not affect your reading, acceleration will and you should *not* accelerate the block! When you say

    i am sure it is uniform acceleration
    I hope you really mean "uniform velocity". It's possible to obtain the friction from a uniformly accelerated block, but then you'd need to know the acceleration. Why make it difficult... just choose a more or less constant velocity (zero acceleration). As you are pulling the block with the scale, you will most likely see the a (slight) oscillation in the rading due to a number of factors, such as varying roughness on the floor, your own imperfection of pulling it with constant strength, and so on. Observe the scale and visually obtain the "average" reading. Do it a couple of times, maybe with different (but in each case constant) velocity, and take the average of your results.
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  5. #4  
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    I think M is right. They are looking for the kinetic coefficient of friction and you have to pull the weight at a steady speed to measure the frictional force. The elastic band is to smooth out the force but it will still jump around a little bit as you accelerate or decelerate the mass, so thats why you average the reading.
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  6. #5  
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    Well the way to find the co-efficient of friction would be to use the equation: Force = (co-eff of friction) x (normal reaction)

    You would work out the force with: F=ma
    Normal reaction would be mass x gravity (9.8 or 10).

    I think... :?
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  7. #6  
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    hey thanks guys,
    i am starting u figure it out.and yes M i meant uniform velocity. how should i measure the force applied. should i just pull the block with elastic band to make my life easier instead of stretching the elastic and then letting the wood block. i am sure should not stretch the elastic and let go the wood because it would then be acceleration. soory my wording is hard to understand.
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  8. #7  
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    Like I said before, the elastic is there to make it easier for you to keep the force, and speed, relatively constant. You could try it with or without the elastic. The answer should be the same. Without the elastic, any speeding up or slowing down from the constant speed will immediately result in a change in measured force. With the elastic, I think you will see a little time lag so you can correct your speed without a huge change in the measured force.
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  9. #8  
    M
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    how should i measure the force applied
    I was assuming you are using a spring scale to pull the block. If you don't have a spring scale, just a simple elastic band you need to be a little more creative. Here is one way to do it: As you are pulling the block across the floor with constant speed, pulling the elastic band horizontally, measure the length of the band (can be a little tricky to concentrate on constant pulling force and read the length from a ruler, but with some practice you should be able to do it). Then take off the band and measure its length without tension. The difference of those lengths is directly proportional to the friction force:

    F = c * (d - d0)

    where d is the length you measured while pulling the block, d0 is the length without tension, and "spring factor" c needs to be obtained by calibration. For calibration, you need to apply a known force (maybe a weight) to your elastic band and measure the extension:

    c = W/(x -d0)

    where W is the known weight hanging on the band, x is the length of the band pulled by the weight, d0 again is the length at zero tension. With a good elastic band, c should be a constant. However, to be on the safe side, make sure your extension applied for calibration is not too different from the extension you see in your experiment.

    Seems too involved? Go buy yourself a spring scale. :wink:

    By the way, if you have no force gauge I am curious how you got the "8 N" measurement in the first place.
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