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: Newtonian Mechanics/Forces Question

  1. Jan 17, 2016 #1
    • Member advised to use the homework template for posts in the homework sections of PF.

    These buckets are being suspended. The cords have no mass and each bucket is 3.5 kg.

    a.) Find the tension in the upper cord (T1) and the lower cord (T2)
    b.) If the apparatus is pulled upwards from the top of the upper cord, what are the new tensions of each cord?

    I understand the first part. The force diagrams consist of:
    (Upper Bucket): T1 upwards, T2 downwards, and Fg downwards
    (Lower Bucket): T2 upwards and Fg downwards

    However, when the apparatus accelerates upwards, the tensions in both cords increases. This makes sense physically but in the force diagram isn't force applied (causing acceleration) going upwards? And if that were the case, then force applied would counter-intuitively decrease the tension within the cords. Can someone graciously clear this up for me?
  2. jcsd
  3. Jan 17, 2016 #2
    Newton's second law applies to each bucket independently.
  4. Jan 17, 2016 #3


    User Avatar
    Education Advisor
    Gold Member

    I'm not sure why you would believe that's the case, but look at ##\sum F = ma##. Solve for the tension, and you'll see why hopefully. You may be confusing systems in equilibrium and accelerating systems while doing the FD.

    Edit, trying to think of a good way to explain what I think is catching you up, but anyway, what's actually accelerating the system upward? The reactionary tension of course! The force that's pulling the system upward itself isn't applied to the buckets, its mediated by the tension in the rope.

    Not sure if that helps or not.
    Last edited: Jan 17, 2016
  5. Jan 17, 2016 #4
    I think I understand! The FA isn't actually an applied force. Well it is, but in terms of this problem it isn't. It's actually just addition upward tensional force on the upper cord FT1! This would, obviously, increase FT1 and in turn increase FT2. Thanks for the help and sorry for not using the proper format of my question. Merci.

    Attached Files:

  6. Jan 17, 2016 #5
    Edit: Ignore those files. I was going to ask a follow-up question but upon creating those images and thinking about the question, it clicked!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted