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Help finding friction force of block moving across horizontal atwood's machine

  1. Apr 23, 2012 #1
    Basically we did a lab where you attach a pulley to the desk with a clamp and then have a friction block (three types: cork, plastic, and felt) attached to a string which is attached to a hanging mass. This basic setup is called an atwood's machine. The purpose of the lab was to identify the coefficient of friction by determining what hanging mass is needed to move the block at a constant speed.
    We did multiple trials with each block, adding different weights to the block and then adding weights to the hanging mass in order to pull the block.
    The steps I need to take in order to find the coefficient of the friction (we're trying to find the kinetic friction by the way) are, I believe, find the normal force (which I found), then find the frictional force, then the coefficient using those two.
    My problem is finding the frictional force, I do not know any formula's which would allow me to find it without the coefficient of friction, which I need the friction force to find.

    I vaguely remember somebody saying that the frictional force is equal to the hanging mass, but I'm not sure that makes sense. Could someone confirm that?

    If the data is needed to explain this to me just let me know.

    If I was to find the force applied by the hanging mass how would that relate to finding the friction force, is that the right step to be taking?
     
  2. jcsd
  3. Apr 23, 2012 #2

    Doc Al

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    Since the speed of the block is constant, the net force on it must equal zero. The only horizontal forces acting on the block are the friction force and the string tension, so they must be equal (and opposite). How does the string tension relate to the weight of the hanging mass?
     
  4. Apr 23, 2012 #3
    Sadly, I don't know, either I was absent or just not paying attention when we went over tension. I don't remember the word ever being mentioned

    also, I'm sorry if I put this in the wrong forum section, I just noticed the homework help section of the forums.
     
    Last edited: Apr 23, 2012
  5. Apr 24, 2012 #4

    Doc Al

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    Well, time to crack open the textbook! :smile: (See: Horizontal Pulley with Friction)

    What forces act on the hanging mass? (Hint: Only two.) What must they add to? (The hanging mass is also moving with constant speed.)
    I will move it.
     
  6. Apr 24, 2012 #5
    Gravity and tension and they must equal 0?
     
  7. Apr 24, 2012 #6

    Doc Al

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    Right. (They must add to zero.)
     
  8. Apr 24, 2012 #7
    Ok, so tension would 9.8 while gravity is -9.8

    so yeah, I'm really clueless as to what to do next, it's extremely frustrating that the concept just won't click.

    I appreciate that you are trying to help me actually learn the concept of this, but the lab report is already late and I'm failing the clss until I hand it in.
    If it's possible for you to just explain how to do this clearly I'd be grateful.
     
    Last edited: Apr 24, 2012
  9. Apr 25, 2012 #8
    Thanks for all the help, I talked to some other people and I believe I know how to do it now.
     
  10. Apr 25, 2012 #9

    Doc Al

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    The weight of the hanging mass is mg, so the string tension must equal mg. And now that you have the string tension, how does that relate to the friction force? (Review post #2.)
     
  11. Apr 25, 2012 #10

    Doc Al

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    Cool. (Sorry I didn't get to this sooner.)
     
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