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: Centre of mass and gravity problem, confused and need help

  1. May 24, 2014 #1
    1. The problem statement, all variables and given/known data

    A uniform plank of length l = 6m and mass M= 90kg rests
    on sawhorses separated by D = 1.5m and equidistant from the
    center of the plank. Cousin Throckmorton wants to stand on the
    right-hand end of the plank. If the plank is to remain at rest, how
    massive can Throckmorton be?


    2. Relevant equations

    3. The attempt at a solution

    I am confused at the diagram. Why is the centre of gravity not at the centre of mass of the plank which should be in the middle ?
  2. jcsd
  3. May 24, 2014 #2
    It is the centre of mass of the system which includes all the masses.
  4. May 24, 2014 #3
    Okay, then how do we find it ?, or what is the explanation that says it should be there ? (referring to the cg in the image)
  5. May 24, 2014 #4


    User Avatar
    Science Advisor

    The center of mass of the plank is at the middle of the plank. Taking x to be the distance from the left end of the plank, that would be x= 3.0. Throckmorton's center of mass is where he is standing at x= 6.0. The center of mass of the system is a weighted average, weighted by their masses: (90*3+ M*6)/(M+ 90) where M is Throckmorton's weight.
  6. May 24, 2014 #5
    Why is it m*6 and not m*3 ? and how do you know where the centre of gravity is ?( cg in the diagram)
  7. May 25, 2014 #6
    He's taking moments about the left end.

    He's giving you the formula for the location of CG
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted