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!

Maximum / Minimum Force

  1. Jan 30, 2009 #1
    1. The problem statement, all variables and given/known data

    An elevator mass 4950 kg is to be designed so that the maximum acceleration is 6.10×10-2g.

    What is the maximum force the motor should exert on the supporting cable?

    What is the minimum force the motor should exert on the supporting cable?

    2. Relevant equations

    [tex]\Sigma[\tex]F = ma

    3. The attempt at a solution

    I know this should be so easy, but Mastering Physics is not liking my answers. I have tried multiplying the combined forces of acceleration and gravity multiplied over the mass to get a large force, that answer was incorrect. I noticed that the units of acceleration is in 'g', which caused me to believe they were referencing that amount multiplied by gravity instead of added to it, which resulted in another answer MP did not like. My results:

    F = 48800 N
    F = 2960 N

    Three significant figures, neither of those answers were correct (this is for Fmax). I am afraid I am missing something really critical, and wanted to get another opinion. Maybe I'm approaching this problem in the wrong way?
     
  2. jcsd
  3. Jan 30, 2009 #2

    Delphi51

    User Avatar
    Homework Helper

    "combined forces of acceleration and gravity" sounds really good! But doesn't lead to the numbers you got. It would be interesting to see the details of your calculation.
    F = ma + mg = ???
     
  4. Jan 30, 2009 #3
    Well mg + ma does equal 48800 N rounded to the number of significant figures the problem asks for.

    The calculation I referred to is: 4950*(g+a), which equals 48900... Hmm...

    Well, I only have one guess left, and I know that 48900 is not correct.
     
  5. Jan 30, 2009 #4

    Delphi51

    User Avatar
    Homework Helper

    Ah, you are using a=0.061! It should be a = 0.061*9.81.
     
  6. Jan 30, 2009 #5
    So it should be calculated as such:

    F = mga + mg = 51500N. Is this right?
     
  7. Jan 30, 2009 #6
    What about finding the minimum force?
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?