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!

Homework Help: Rotational force

  1. Jun 14, 2008 #1

    My question is this:
    A mail bag with a mass of 105 kg is suspended by a vertical rope of length 7.00 m. What horizontal force is necessary to hold the bag in a position displaced sideways a distance of 1.00 m from its initial position?

    I tried to start this problem but I keep getting stuck. I start out figuring that it is an isosceles triganle with the legs being 7m and the base being 1 m. from there i figured that if i resolved the 1 meter into its horizontal and vertical components, I would be able to find the force using Newtons second law using 9.8 meters per second squared as the acceleration and the mass as 105 kg. but i think that using 9.8 meters per second squared is wrong because that is acceleration due to gravity and I only need horizontal acceleration....... I am confusing myself with this problem.......
  2. jcsd
  3. Jun 14, 2008 #2


    User Avatar
    Science Advisor
    Homework Helper

    Welcome to PF!

    Hi emeraldempres ! Welcome to PF! :smile:

    I think the 1.00 m is the sideways distance, not the total distance.
    There is no acceleration!

    The situation is static.

    Just use a force diagram! :smile:
  4. Jun 14, 2008 #3
    thanks for the welcome!!!

    so if i do the diagram, i have two ropes, one hanging vertically (that is slanted) and one pulling horizontally, and I am calculating the tension on the one pulling horizontally? how do i know the angles at which the semi vertical rope is slanted?am i on th right track? Thanks for your reply =)
  5. Jun 15, 2008 #4


    User Avatar
    Science Advisor
    Homework Helper

    Hi emeraldempres! :smile:

    Yes (and the weight should also be in the diagram, of course).
    Geometry … you have a right-angled triangle. :smile:
  6. Jun 15, 2008 #5
    thank you. I have come to the answer of 149 Newtons. The second part of the problem says to calculate the work done by the worker to get to this postion. I have tried work= 149N*1m and that does not work, then w= 149n*1m*cos (82 degrees), and that does not work. is ther something i am missing?
  7. Jun 16, 2008 #6


    User Avatar
    Homework Helper

    Hi emeraldempres,

    The work done is related to the change in energy. How is the energy changing as the worker pulls the mailbag to its final position?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook