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Center of mass

  1. Mar 30, 2015 #1
    1. The problem statement, all variables and given/known data
    Fre4Jn7.png

    2. Relevant equations


    3. The attempt at a solution
    I chose the answer A, but it's not correct, can anyone explain why?
     
  2. jcsd
  3. Mar 30, 2015 #2

    robphy

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    Is there a better answer?
     
  4. Mar 30, 2015 #3
    yes, B

    but why?

    If I hold a chain thats 4 meters long, from one end. I can say the center of mass of the chain is 2 meters below my hand.

    I shouldn't say the center of mass of the chain is right below my hand, can I?
     
  5. Mar 30, 2015 #4

    robphy

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    This object is subject to two forces...
    gravitation (which can thought of as applied all at the center of mass) and
    the normal force applied by the fulcrum (applied at the point of contact).
    When balanced, the sum of the forces on the object is zero and the sum of the torques* on the object is zero [*torques taken about any point].
    When balanced, where must the center of mass be in relation to the point of contact?

    Among the choices in the posed question, the "end" in A could refer to the extreme position of the bat.
     
  6. Mar 30, 2015 #5

    jbriggs444

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    "directly above" in this problem is intended to mean "straight above" i.e. above and not offset a little to one side or the other.
     
  7. Mar 30, 2015 #6
    so it wouldn't count if I held the bat? It has to be ONLY balanced?
     
  8. Mar 30, 2015 #7

    robphy

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    The question says "balance[d]... on top of your finger" (implying a single point of contact).
     
  9. Mar 30, 2015 #8
    ok, just making sure. makes sense since you can't hold up a chain with using just 1 point of contact.
     
  10. Mar 30, 2015 #9

    jbriggs444

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    Sure you can. But how is that relevant?
     
  11. Mar 30, 2015 #10
    yes, actually you can, if you put your finger under one of the 'holes' , then would I say the center of mass is still directly above my finger?
     
  12. Mar 30, 2015 #11
    lets say I balanced a ice cream cone on the floor:
    L3H6ZDm.png

    would the center of mass still be at the point where the floor and cone contact?
     
  13. Mar 30, 2015 #12

    jbriggs444

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    The center of mass of a chain hanging at rest below your finger is directly below your finger.
     
  14. Mar 30, 2015 #13

    jbriggs444

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    The center of mass would be directly above the point at which the floor and cone contact. Refer to post #5 for a definition of "directly above".
     
  15. Mar 30, 2015 #14
    i understand now, ty
     
    Last edited: Mar 30, 2015
  16. Mar 30, 2015 #15

    robphy

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    Just to be clear... (because I don't think that this point was explicitly made)...
    The center of mass of an object is determined by the positions and masses of the all of the bits that compose the object.
    If the object is rigid, that location with respect to the object doesn't change.... no matter where you place the object, how you orient the object, or apply forces to the object.

    When an object is balanced against gravity by a single contact force,
    the point of contact and the center of mass must be on the same vertical line.
    (In your balanced cone example, the center of mass is located near the middle of the cone-and-ice-cream... [vertically] above the point of contact on the floor
    ... but not at the point of contact on the floor. Similarly, for the solid bat.)
     
  17. Mar 30, 2015 #16
    so there can be an even better answer : the center of mass of the bat is directly above my finger, and at the heavier, thicker end of the bat, correct?
     
  18. Mar 30, 2015 #17

    jbriggs444

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    No.

    The center of mass will not neccessarily be at the heavier, thicker end of the bat. But it will not be at the midpoint of the bat either. It will be closer to the heavy end than to the light end. Its position is (in a particular sense), the "average" position of the mass in the bat.
     
  19. Mar 30, 2015 #18

    robphy

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