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Magical Pulley? I think so.

  1. Nov 22, 2008 #1
    Basically, I'm trying to gain a true understanding of what force actually is. I've done the atwood machine experiment at my college's physics lab and studied energy/power/work, but I still don't have a grasp of what force actually is beyond just calculating it.

    And so, I went on an adventure to recreate the experiment using stuff from home depot and walmart:




    The difference in mass was 5 grams. It took the metric spring scale 5 seconds to travel from the vertical center of the pulley to the floor, a distance of 146.5 cm. I calculated final velocity to be 0.586 m/s. Acceleration 0.1172 m/s^2.

    This is what I don't understand: The lighter object pulls downward on the cord with a force equal to its own mass times gravity. The heavier object pulls downward with a slightly larger force, also its own mass times gravity.

    The difference in force causes the heavier object to be pulled toward the center of the earth... but if this was just a 5 gram object, it would have accelerated at 9.8 meters per second squared.

    I just don't get how we can actually change acceleration by linking two masses like that ><
  2. jcsd
  3. Nov 22, 2008 #2


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    I don't actually understand the set-up here. An Atwood machine is simply two masses connected by a string the passes over a pulley.
  4. Nov 22, 2008 #3
    The spring scale is not measuring anything. It's just a mass, as is the snap hook. The spring scale is 46 grams. The metal snap hook is 41 grams. The spring scale started from rest and descended a certain distance in 5 seconds. A white cord connected the two objects over a pulley attached to a vent in my ceiling.
  5. Nov 22, 2008 #4
    Nice experiment :smile:

    The difference between your two scenarios is that you now have a bigger mass, not just 5 grams. A bigger mass is more inertia.

    Let's say one of the masses (on one side of the pulley) is M1 = m grams and your other mass M2 is m+5 grams. Let's further consider M2 to be made of two pieces: an 'm' gram piece, and a 5 gram piece. The 5 gram piece by itself would accelerate at 9.8m/s^2; and the remaining m grams by itself would stay stationary (because it is being balanced by M1). But now we have glued the two 'pieces' together -- so they cannot accelerate at the same rate. The 5 gram piece tries to accelerate faster, but the m gram piece tries not to move; so they reach a compromise.
  6. Nov 22, 2008 #5


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    So you don't understand the physics behind the Atwood Machine, this has nothing to do with your experimental set-up, right?
  7. Nov 22, 2008 #6
    Ahhhh.... it's just hard to understand from an intuitive perspective, but I think I get it now.

    The purpose of the pulley experiment is to apply a truly constant force to an object in a way we could not normally do with just a spring or our own hand, so that we can prove the existence of "an object at rest tends to stay at rest" right?
  8. Nov 22, 2008 #7
    I understand the math, and I can calculate crap all day long on a test when asked. The point of this was to gain a logical understanding of what was actually going on. To quote Feynman again: "The beauty of math is that you don't know what you're talking about."

    Well, I wanna know.
  9. Nov 22, 2008 #8


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    An alternative viewpoint would be to consider the forces acting on each mass, as I'm sure you have done in class. If the only force acting on each mass was the weight of the mass, then of course they would accelerate a g towards the earth. However, in the case of the Atwood machine, there is an additional force acting on each mass, that of the tension in the string.
    In essence yes, Atwood machine was developed as a purely educational demonstration to illustrate the laws of uniformly accelerated motion.
  10. Nov 22, 2008 #9
    Awesome, thanks man.

    brb preparing bathtub for buoyancy experiment...
  11. Nov 22, 2008 #10
    Well, what I said was simply one possible way to think about the experiment - it is only partially correct (there are actually 3 pieces "glued" together, the 5 grams, the m grams, and the mass M1, which is attached by the string).

    Anyway, the purpose of an Atwood is not merely to prove Newton's first law. It changes directions of forces, and is therefore very useful. You should probably read a book on that.

    Do your experimental numbers make sense?
  12. Nov 22, 2008 #11
    To be perfectly honest, the numbers never make sense to me. The numbers are right for testing purposes, but as far as truly understanding what is happening, it's always foggy.

    I'm listening to Feynman's '61 lecture, and it's really helping. I think that's the only resource I've found where someone teaches to students and not to peers (oh, and this web site helps a great deal of course).
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