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I need to get a 100 on this last Physics lab assignment, but I can't get this rod

  1. Nov 30, 2012 #1
    I need to get a 100 on this last Physics lab assignment, but I can't get this "rod"

    ...problem :(
    Help would be appreciated

    Number2Physics.png

    (a) L1 = (65 cm)/2 = 32.5 cm

    "CORRECT"

    (b) [(149 kg)(1) + (149 kg)(65 cm) + (149 kg)(65 cm + 32.5 cm)] / (3)(149 kg)
    = (149 + 9685 + 14527.5) / (447)
    = 54.5

    Then,

    54.5 - 32.5 = 22 cm

    "INCORRECT"

    -.- Help?

    (c) ????

    L3 _________
     
  2. jcsd
  3. Nov 30, 2012 #2
    Re: I need to get a 100 on this last Physics lab assignment, but I can't get this "ro

    What's the force the string from 1 exerts on 2, and where is that force applied?
     
  4. Nov 30, 2012 #3
    Re: I need to get a 100 on this last Physics lab assignment, but I can't get this "ro

    (149)(65)? in the vertical direction.
     
  5. Nov 30, 2012 #4
    Re: I need to get a 100 on this last Physics lab assignment, but I can't get this "ro

    Well, that's torque. I just want to know if you held the string with rod 1 and its masses attached, instead of attaching it to the rod, how much force would be pulling down on your hand (ignoring the weight of your own hand)?
     
  6. Nov 30, 2012 #5
    Re: I need to get a 100 on this last Physics lab assignment, but I can't get this "ro

    149(9.81)?
     
  7. Nov 30, 2012 #6
    Re: I need to get a 100 on this last Physics lab assignment, but I can't get this "ro

    aren't there 2 masses on rod 1?
     
  8. Nov 30, 2012 #7
    Re: I need to get a 100 on this last Physics lab assignment, but I can't get this "ro

    Oh...
    (149+149)(9.81)
     
  9. Nov 30, 2012 #8
    Re: I need to get a 100 on this last Physics lab assignment, but I can't get this "ro

    Okay, now we can ignore 9.81 because it will be in every term.

    But my point is, you can model rod 2 as having 1m (149 kg) at one end and 2m (298 kg) at the other.

    So find the center of mass of that system for Rod 2.
    Rod 3 can be modeled in a similar way, but you now have 3m on one end.
     
  10. Dec 1, 2012 #9
    Re: I need to get a 100 on this last Physics lab assignment, but I can't get this "ro

    [(149 kg)(1 m) + (298 kg)(2 m) + (447 kg)(3 m)] / (3 m)(447 kg)

    Like that???
     
  11. Dec 1, 2012 #10
    Re: I need to get a 100 on this last Physics lab assignment, but I can't get this "ro

    Eep, no. m as in m=mass=149 kg.

    Rod 3 has 3*149 kg at one end, and 149 kg at the other. Because all of rod 2 and rod 1's mass (a total of 3*149 kg) is supported by one string at the end of rod 3.
     
  12. Dec 1, 2012 #11
    Re: I need to get a 100 on this last Physics lab assignment, but I can't get this "ro

    Ok so the equation should be...

    [(149 kg)(1) + (149 kg)(65 cm)(2) + (149 kg)(65 cm + 32.5 cm)(3)] / (3)(149 kg)??

    Like that?
     
  13. Dec 1, 2012 #12
    Re: I need to get a 100 on this last Physics lab assignment, but I can't get this "ro

    No, it should be the mass*distance at the one end + mass times distance at the other end, divided by total mass.

    Or [149 kg * (3) + 149 kg *(1) * 65 cm] / 4 * 149kg

    the diagram shows that L3 is the distance from the end of that rod to the balance point, so you don't need to add anything.

    If you do the algebra, you should find that because the masses are equal, if you have 3 on one side and 1 on the other, the balance point is just 1/4 L. for rod two, it's 1/3 L. So you kind of stumbled into the answer the wrong way.
     
  14. Dec 1, 2012 #13
    Re: I need to get a 100 on this last Physics lab assignment, but I can't get this "ro

    [149 kg * (3) + 149 kg *(1) * 65 cm] / 4 * 149kg


    ^

    That still doesn't give me the right answer though :(
    I just submitted 17 cm and it still marked it as incorrect!

    I did:

    447.0000 + 9685.0000 / 596.0000

    10132.0000 / 596.0000

    17.0000

    ??????
     
  15. Dec 1, 2012 #14
    Re: I need to get a 100 on this last Physics lab assignment, but I can't get this "ro

    You are right, my bad. The first term is at 0 cm. So

    [149 kg * (3) * 0 cm + 149 kg *(1) * 65 cm] / 4 * 149kg
     
  16. Dec 1, 2012 #15
    Re: I need to get a 100 on this last Physics lab assignment, but I can't get this "ro

    I did

    0 + 9685.0000 / 596.0000

    = 16.2500

    I put 16.25 cm in the answer box and I still got it wrong :(
    I've already used 3 submissions...anything beyond 4 with be .33 points off :(
     
  17. Dec 1, 2012 #16
    Re: I need to get a 100 on this last Physics lab assignment, but I can't get this "ro

    this is part c, right?

    If 16.25 cm isn't the answer to part c, then I have no idea what's going on.
     
  18. Dec 1, 2012 #17
    Re: I need to get a 100 on this last Physics lab assignment, but I can't get this "ro

    This is part c yeah...
     
  19. Dec 1, 2012 #18
    Re: I need to get a 100 on this last Physics lab assignment, but I can't get this "ro

    wait no this is part b!!
     
  20. Dec 1, 2012 #19
    Re: I need to get a 100 on this last Physics lab assignment, but I can't get this "ro

    ohh you were doing part c?
     
  21. Dec 1, 2012 #20
    Re: I need to get a 100 on this last Physics lab assignment, but I can't get this "ro

    oh 16.25 for part c is CORRECT nvm...

    I was asking for part b
     
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