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

Center of mass, forces and momentum

  1. May 19, 2006 #1
    1) A force F = A/t^2 acts on an object. At time t = t_o its momentum is 0. What is its momentum after a very long time?
    First I used Newton's second law:
    dP/dt = A/t^2
    t^2 dP = A dt
    dP = A/t^2 dt
    And I took the integral of both sides, the left from 0 to P, the right from t_o to t...
    P = -A/t + A/t_o
    So as t-> infinity, P -> A/t_o. That's what I got. Is this correct?

    2) Picture: http://www.brokendream.net/xh4/com.JPG
    A wire is bent like a triangle with side lengths d at 45 degrees from the horizontal.
    What is the y co-ordinate of the center of mass?
    What I thought to do is to treat the center of mass as the centroid, taking the elements to be the length of each wire.
    So I did:
    y_com = [d*(d sin 45)/2 + d*(d sin 45)/2]/(2d) = (d sin 45)/2 = 0.35355d.
    Is this correct?

    3) Picture: http://www.brokendream.net/xh4/kidswing.JPG
    With what force must the child pull down on the string if the combined mass of the child and the swing is W? Neglect friction from the pulley.
    I'm guessing, from F = ma, 2T - W = 0 and thus T = W/2 is the force?

    Thank you.
    Last edited by a moderator: Apr 22, 2017
  2. jcsd
  3. May 19, 2006 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    I'm not sure about your method here, why have you attempted to seperate variables when there is only one? (I'm assuming A is a constant) In which case it would be sufficent to do;

    [tex] P = \int \frac{dP}{dt} \; dt = \int \frac{A}{t^2} \; dt[/tex]

    And evaluate it. Also, I think your question may require a qualative answer, such as; "When t is very large, the momentum is very small".

  4. May 19, 2006 #3


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Spot on, vector sum of force must equal zero.

    Last edited by a moderator: Apr 22, 2017
  5. May 19, 2006 #4
    Dunno, guess I'm just used to doing this from the way my physics teacher does it. :P
    Anyway, it is a quantitative answer. (It was on a test)
  6. May 19, 2006 #5


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Ahh, sorry I didn't see this line;

    I thought you where trying to integrate this
    for a moment :confused:. Anyway, I worked through it and my answer agrees with yours.

  7. May 19, 2006 #6
    Yes Pseudostatic, your method and answer are absolutely correct.

    Yes Hoot, that's exactly what the OP has done. The additional term in his final expression is the value of the constant of integration which can be evaluated by letting t= t_0 and thereby P= 0
    For the final answer as the OP did let t -> inf. and u know the rest.


    Edit: Ah I guess it was just a small misunderstanding there .
  8. May 19, 2006 #7
    Anyone know about my center of mass question? :)
  9. May 19, 2006 #8


    User Avatar
    Homework Helper

    This is not a triangular sheet and hence the center of mass will not be at centroid. What is the ratio of the mass of three sides?
  10. May 19, 2006 #9

    Doc Al

    User Avatar

    Staff: Mentor

    If your wire "triangle" just has two sides (the top two), then this is correct.

    (What you did has nothing to do with the centroid of a triangle though. :smile: )
    Last edited by a moderator: Apr 22, 2017
  11. May 19, 2006 #10
    Well, yeah, it's just those two sides. :D
    Great.... thanks. Now I know I got atleast 3 multiple-choice questions right on my physics exam. :)
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook