1. Not finding help here? Sign up for a free 30min 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!

Forces on a pail being lowered/pulled up, with/without water

  1. Oct 3, 2011 #1
    A homework question mostly dealing with FBD, but i missed a week of class so im a bit behind..

    A pail is suspended from a vertical rope, wrapped around a winch, so that the pail can be raised or lowered inside a well.

    a)The weight of the empty pail is 4N. Suppose the pail is descending vertically with increasing speed. Draw a FBD for the pail. Is the tension in the rope larger than, smaller than, or equal to 4N? Explain.

    b)After being filled with water, the full pail has a weight of 20N. When this full pail is being pulled vertically up with constant speed. is the tension in the rope larger than, smaller than, or equal to 20N? Explain your reasoning.



    Attempt at solution:

    a) for the FBD, i can only think of two acting forces, tension and weight, in opposite directions. Because the pail is being lowered with increasing speed, does that mean the Fg > Ft, thus the tension in the rope is less than 4N?


    b)When the pail is full and being pulled up, will that mean the opposite, Fg < Ft?


    I know its a simple question, and im probably over thinking it, but itd be great if i could be certain!
     
  2. jcsd
  3. Oct 3, 2011 #2

    PeterO

    User Avatar
    Homework Helper

    You were correct in (a) and incorrect in (b), indicating it may have been a lucky guess in (a)

    For (a), in what direction is the acceleration? In what direction is the net force? What does that mean for Fg and Ft?

    For (b), in what direction is the acceleration? In what direction is the net force? What does that mean for Fg and Ft?
     
  4. Oct 3, 2011 #3
    a) The acceleration is down, but is the net force up because it is equal to tension?

    b) I think i see where i made the mistake, the pail, now full of water, is pulled up at a constant speed. So then Fg = Ft.

    But its still unclear to me how tension, force of gravity and net force are related? Ive read other threads and im still unsure..
     
  5. Oct 3, 2011 #4

    PeterO

    User Avatar
    Homework Helper

    (a) F = ma tells us that Force and acceleration are in the same direction, so the net force is down. That means Fg exceeds Ft as you said. I thought you may have guessed.

    (b) you are now correct Fg = Ft.
     
  6. Oct 3, 2011 #5

    PeterO

    User Avatar
    Homework Helper

    Tension and Force of gravity are the actual forces acting on the object. They are vectors.
    The vector sum of the two give the net force.
    The Net force is the F in Newtons 2nd Law. F = ma.

    When the bucket accelerates down, Fg > Ft so that the net force is down.
    eg. if Fg is 3N down, and Ft is 2.5N up, the net force is 0.5N down.

    When the bucket is acceleratng up, Fg < Ft so that the net force is up
    eg if Fg is 3N down, and Ft is 4 N up, then net force is 1N up.

    If the bucket is not accelerating, Fg = Ft, so that the net force is zero.
    eg if Fg is 20N down and Ft is 20N up, then the net force is zero.
     
  7. Oct 3, 2011 #6
    Thank you PeterO, i believe i understand now, for B) even though the pail is moving up, it is at a constand speed so Fg-Ft = Fn. Fn=ma=0 so Fg=Ft!! Thank you!!
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Forces on a pail being lowered/pulled up, with/without water
  1. Chain being pulled up (Replies: 12)

Loading...