Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: 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


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    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


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    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.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook