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: 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

    2. Relevant equations
    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


    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:
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook