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Homework Help: Newton law's/ force related

  1. Oct 26, 2008 #1
    1. The problem statement, all variables and given/known data

    A crate of books is to be put on a truck with
    the help of some planks sloping up at 31◦. The
    mass of the crate is 70 kg, and the coefficient
    of sliding friction between it and the planks is
    0.4. You and your friends push horizontally
    with a force ~F .
    The acceleration of gravity is 9.81 m/s2 .
    Once the crate has started to move, how
    large must F be in order to keep the crate
    moving at constant speed? Answer in units of
    kN.


    2. Relevant equations
    Fnet=ma
    u*F= Ff


    3. The attempt at a solution

    i did .4 x F = sin(31) x 70 x 9.81, solving for F i get 884.192 N/ 1000
    = .884192 kN
    however it's wrong
    so im confused as how to approach this problem

    i tried cos(31) as well
    and i've tried these solutions; all of which were rejected ( on the UT website )
    10/26/08 8:46 PM Try#1: .353677
    10/26/08 9:46 PM Try#2: 141.471
    10/26/08 9:46 PM Try#3: .141471
    10/26/08 9:47 PM Try#4: .235447
    10/26/08 9:48 PM Try#5: .884192
    10/26/08 9:49 PM Try#6: 1.47154

    also i apologize if i posted in the wrong forum/ had bad formatting

    oh nevermind lovely physics forum~
    i got the answer
    for anyone who's interested heres the work process
    (excuse my intangible markings i was doing this on another forum as well)

    ma = Fpush - Fg - Ff
    Ff = .4*(cos31)*686.7 = 235.4
    Fg = 686.7*sin31 = 353.7
    a = 0

    0 = Fpush - 353.7 - 235.4
    Fpush = 589.1 N

    edit: Ah crap it's 9.81 ~_~
    edit2: ah crap you're pushing horizontally

    Fpush = Fhor - Ff
    589.1 = Fhor*cos31 - .4*Fhor*sin31
    589.1 = 0.651*Fhor

    Fhor = 905 N
    or .905 kN


    i would still appeiciate any form of physics whatsoever, tips and help drawing digrams, overall picture, mindset, some practice problems - recommend books/ websites
    the main goal for me is to get a 4,5 on the AP physics C exam
     
    Last edited: Oct 26, 2008
  2. jcsd
  3. Oct 27, 2008 #2

    tiny-tim

    User Avatar
    Science Advisor
    Homework Helper

    Welcome to PF!

    Welcome to PF! :smile:

    Tip: if you were systematic in writing the proof, you wouldn't have missed the F sin31º the first time.

    You should always state which direction you're taking components in (and why) … don't just do it in your head, write it down

    in this case I can see that you took components in the normal direction, so you should have started by saying:

    "there is no acceleration in the normal direction, so the sum of the components of force in that direction is zero … " :smile:
     
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