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!

Force and Acceleration

  1. Apr 8, 2009 #1
    1. The problem statement, all variables and given/known data
    At a given instant block A of weight 10-lb is moving downward with speed 6 ft/s. Determine its speed at a later time t = 2s. Block B has a weight 4lb and the coefficient of kinetic friction between it and the horizontal plane is μ = 0.2. Neglect the mass of the pulleys and cord.


    T = 3.385 [tex]a_{A}[/tex] = 10.403 [tex]ft/s^{2}[/tex] [tex]a_{B}[/tex] = - 20.806 [tex]ft/s^{2}[/tex]

    not sure if these are correct seeing i get different answers.

    2. Relevant equations

    T - [tex]\mu_{k}w_{B}[/tex] = [tex]\frac{-w_{B}}{g}a_{B}[/tex]

    2T - [tex]w_{A} = (\frac{-w_{A}}{g}) (-a_{A})[/tex]

    [tex]2a_{A} + a_{B} = 0[/tex]

    3. The attempt at a solution

    since [tex]2a_{A} + a_{B} = 0[/tex] i'd solve for [tex]a_{A}[/tex] and [tex]a_{B}[/tex] in the first 2 equations and sub into the third one

    [tex]2(\frac{2Tg}{w_{A}} - g) + (\frac{-Tg}{w_{B}} + \mu_{k}g)[/tex] = 0

    then i solve for T

    [tex]\frac{2g - \mu_{k}g}{g(\frac{4}{w_{A}} - \frac{1}{w_{B}})}[/tex] = T

    T= 12N

    then sub back into one of the first 2 equations to solve for either [tex]a_{A}[/tex] or [tex]a_{B}[/tex].

    [tex]a_{B}[/tex] = -90.16[tex]ft/s^{2}[/tex]
    [tex]a_{A}[/tex] = 45.08[tex]ft/s^{2}[/tex]

    any help would b nice
    Last edited: Apr 9, 2009
  2. jcsd
  3. Apr 8, 2009 #2


    User Avatar
    Homework Helper





    Unit potpourri is never a good recipe.
  4. Apr 9, 2009 #3
    it was late...i changed the units, i was just used to [tex]m/s^{2}[/tex] from previous physics courses....
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Force and Acceleration
  1. Force and Acceleration (Replies: 0)

  2. Force and Acceleration (Replies: 5)