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: Modified Block Problem -- Block on top and on the side

  1. Dec 28, 2017 #1
    << Thread moved to the schoolwork forums from the technical forums, so no Homework Template is shown >>

    The problem is a modified version of two basic block problems. This is my work so far, but I end up with two equations for acceleration from m1 and m2, I don't understand how to use both equations. So my questions are -

    1) Did I define the problem correctly to begin with?

    2) Which acceleration value do I plug in for the final expression for F? and why?

    3) Overall, is my derivation correct?

    This is not a hw problem. I just made it up to test my conceptual understanding.


    Last edited by a moderator: Dec 28, 2017
  2. jcsd
  3. Dec 28, 2017 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2017 Award

    I think this should be in the Homework section. (it is a "homework style" problem)
    I'll take a look at your work, but you are going to need 2 acceleration inequalities. Acceleration will need to be greater than or equal to some threshold value to keep m2 from sliding down. Also, acceleration will need to be less than or equal to some other threshold value so that m1 does not start sliding. So you would have a range of accelerations which would satisfy.

    It could be possible that there is only one critical value of acceleration which satisfies both. Or it could work out that the 2 inequalities do not intersect, and there is no solution.
    Last edited: Dec 28, 2017
  4. Dec 28, 2017 #3


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Further to scottdave's reply, there are several errors in the problem statement.
    μs.. is not a force, it is a coefficient of friction. If the normal force is N, and there is no slippage, and the actual frictional force is Fs then the formula is |Fs|≤μsN. Note the ≤, not =.
    As scottdave writes, this leads to two constraints on the applied force, F, which might or might not allow some range of solutions.
    And on a pedantic note, you mean that the two smaller masses do not move relative to M.
  5. Dec 28, 2017 #4


    User Avatar
    Homework Helper
    Gold Member

    I think the total force on M in the x-axis should be ##F-F_{sM}-F_{m_2M}-F_{sm_1}##. Other than that you should replace some equalities with inequalities. for example I think it should be ##F_{sm_1}\geq m_1a##

    Nice problem you made (and also the solution is very good , with detailed schemes and equations). It remembered me of a similar problem my teacher made up when I was in highschool (about 30 years ago). I gave you a like for this :D.
    Last edited: Dec 28, 2017
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted