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!

Dynamics Pushing Blocks

  1. May 21, 2014 #1
    Please look at pictures.
    Weirdly, my textbook does not give the answer to this question.

    Can you check my answer? Sorry for the really bad writing. If you ask, I can more clearly write it.
     

    Attached Files:

    Last edited: May 21, 2014
  2. jcsd
  3. May 21, 2014 #2

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    The handwriting is decipherable, but it's hard to follow what you are doing because it's not clear which equations relate to which FBD.
    Please post your working by typing - it's not hard - making it clear which forces relate to which body.
    Click on the Go Advanced button. You can pick ∑ off the quick symbols on the right and superscript and subscript from the X2, X2 icons along the top.
     
  4. May 21, 2014 #3
    Here goes nothing!
    For m1:
    ∑Fy=N1-m1g-sin(30)F=0
    ∑Fx=-N21-fk+cos(30)F=m1a

    For m2:
    ∑Fx=-μkm2g+N12=m2a
    ∑Fy=N2-m2g=0

    Add both ∑Fx equations:
    km1g-μkm2g+cos(30)F=(m1+m2)a

    a=1.41 m/s^2
    N12=N21=11.52N

    That looks great! How about the physics?
     
  5. May 21, 2014 #4

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    You've missed something out. Write out the equations relating N1, F, fk.
     
  6. May 21, 2014 #5
    I do not understand. Can you be more specific? What is it I am missing? I've been looking at the problem long enough that everything seems good.
     
  7. May 21, 2014 #6
    Got it. I mixed up the fk equation. I'm fixing it now and posting the new solution.
     
  8. May 21, 2014 #7

    Here is my new equation:

    μk(sin(30)F+m1g)-μkm2g+cos30F=(m1+m2)a

    a=1.16 m/s^2
    N12=N21=10.52 N
     
  9. May 21, 2014 #8

    Nathanael

    User Avatar
    Homework Helper

    Those answers are correct (and if they're not, we made the same mistakes hahah)

    Except for the rounding, if you rounded a to 2 decimals it would actually go up to 1.17 but that's pedantic, your equation is correct.



    edit: Actually, there should be a negative sign in very the beginning of your first equation, but you must have just forgotten to type that, because your answer is correct.
     
  10. May 21, 2014 #9
    Yes, I did forget the minus! I hope someone else confirms! At least we are two ;)
     
  11. May 22, 2014 #10

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Make that three.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Dynamics Pushing Blocks
  1. Pushing blocks (Replies: 2)

  2. Pushing blocks (Replies: 6)

  3. Pushing on a Block (Replies: 8)

Loading...