1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Pulling at an angle

  1. Jul 3, 2011 #1
    1. The problem statement, all variables and given/known data
    A box of mass 30kg is being pulled along rough horizontal ground at a constant speed using a rope. The rope makes an angle of 20 degrees with the ground. The coefficient of friction between the box and the ground is 0.4. The box is modeled as a particle and the rope as a light, inextensible string. The tension in the rope is P newtons.

    2. Relevant equations

    3. The attempt at a solution

    The normal force is
    [tex]30g-P\sin 20^\circ.[/tex]
    The force of friction is
    [tex](30g-P\sin 20^\circ)0.4,[/tex]
    which should be equal to the pulling force,
    [tex]P\cos 20^\circ.[/tex]
    Hence
    [tex](30g-P\sin 20^\circ)0.4=P\cos20^\circ.[/tex]
    Taking g as 9.8 and solving the equation yields
    [tex]P=109,[/tex]
    corrected to 3 significant figures.

    However, the answer says that it should be 125 instead of 109. What did I do wrong? Also, the book seems to prefer to have 3 significant figures but 9.8, the numerical value of g, only has 2, which seems weird to me. Any justifications for it?

    (Sorry, I don't know how to type the degree sign in LaTeX. Can anyone teach me how to?

    EDIT: added the degree signs
     
    Last edited: Jul 4, 2011
  2. jcsd
  3. Jul 3, 2011 #2

    rock.freak667

    User Avatar
    Homework Helper

    I would say that you are correct. Even putting g = 10 N/kg would not get it to 125 N.
     
  4. Jul 3, 2011 #3

    SammyS

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    20^\circ

    [tex](30g-P\sin 20^\circ)0.4=P\cos20^\circ[/tex]
     
  5. Jul 3, 2011 #4
    I am also getting 190 N.

    You can also do degrees by making a lower case o into a superscript with the x2 button at the top of the message composing window: cos20o
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Pulling at an angle
Loading...