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Ropes Pulleys Help! Exam 2morrow!

  1. Jan 30, 2005 #1
    Ok, if you had a question where a cart (m1) is resting on a surface, and is attached to a string passing over a frictionless pulley, off of which another mass (m2) is suspended in the air, what is the equation that I use?

    It is like an atwood machine, except m1 is on a surface, and, I need to take into account the friction affecting m1.

    it is hard cause my tect book only shows me how to derive an eqution from
    Fnet = ma when there is no friction on the surface. But I know it will have friction in the exam.
     
  2. jcsd
  3. Jan 30, 2005 #2

    quasar987

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    You use the one and only equation governing all of mechanics. [itex]\Sigma \vec{F} = m\vec{a}[/itex].

    Is your problem that you don't know what the form of the frictional force is?

    Post your work if you need assistance. And also post exactly what the question is; i.e. what do they want you to find and what do you know about the initial conditions.
     
  4. Jan 30, 2005 #3
    Solved it! I think.

    For the Mass on the Surface:

    a = (T - Ff) / m1

    For the Mass suspended in the air:

    a = (Fg - T) / m2

    I am not sure which is correct, is a = (Fg - T) / m2 correct and you don't take m1 into account because the force on that is caused by T which is already in the equation? or must you put it in the equation like that of an Atwood Machine?

    a = (Fg - T) / (m1 + m2)


    Atwood Machine:
    a = (m2-m1)g / (m1 + m2)

    I have a Feeling that the First 2 are correct because I found them in my notes, however I could have copied them incorrectly.
     
  5. Jan 30, 2005 #4

    quasar987

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    Yes, these are the correct equations of motion. You may combine them to find the tension T, knowing from experience that the acceleration a, is the same for both block.
     
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