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Physics question

Discussion in 'Off-Topic Discussion' started by svdude, Apr 12, 2019.

  1. Apr 12, 2019 at 9:04 PM
    #41
    Skydvrr

    Skydvrr Well-Known Member

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    Nah
     
  2. Apr 12, 2019 at 9:11 PM
    #42
    Inferno!

    Inferno! Well-Known Member

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    Need to know the weight of the cable. The shorter cable carries more tension from the beam, and the longer cable carries additional tension cause of the increased cable mass.

    Registered engineer here: where do I send my invoice? :D
     
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  3. Apr 12, 2019 at 9:26 PM
    #43
    Toyko Joe

    Toyko Joe Here for the pictures

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    I've been away from this type of problem for a few years but I think the cables can't be 90* to the ceiling. I think the load would balance between the two connections slightly shifting the beam to the left because gravity is acting on the entire beam.


    Cable on the left would have more tension than the right cable because it's holding the weight of the beam and the force of the different lenth of cable causing the beam to shift towards the left.



    Yellow lines are showing what I think the cables would actually do, blue is the beam and the purple is to show something...



    IMG_2640.PNG.jpg
     
  4. Apr 12, 2019 at 9:51 PM
    #44
    Fuergrissa

    Fuergrissa Well-Known Member

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    F931CE42-17C5-4B56-8474-16201375B375.jpg


    Op in this case, you can neglect the mass of the cable for you situation as the load is small.

    But if your in a windy area, we’re guna have to start considering vortex induced vibrations.
     
    Last edited: Apr 12, 2019
  5. Apr 12, 2019 at 10:18 PM
    #45
    Skydvrr

    Skydvrr Well-Known Member

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    How close is it to the event horizon?
     
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  6. Apr 12, 2019 at 10:34 PM
    #46
    hiPSI

    hiPSI Laminar Flow

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    Beyond the Event Horizon....
     
  7. Apr 12, 2019 at 10:37 PM
    #47
    svdude

    svdude [OP] Well-Known Member

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    So I'm going to make a hoisting system for my garage and this problem is to be sure the load stays level. Imagine lifting this beam (weight may be upwards of 200 lbs) using a double or triple pulley system on both ends of the beam and one single cable through both pulley systems. If in the problem I posted, the high side of the beam is essentially the heavy side, then with a pulley system the load will always self level when raising or lowering the load.
     
  8. Apr 12, 2019 at 10:45 PM
    #48
    BillyToy

    BillyToy Well-Known Member

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    This better not be for one of them damn rooftop treehouses we been fighting over...
     
  9. Apr 12, 2019 at 10:50 PM
    #49
    svdude

    svdude [OP] Well-Known Member

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    Actually it's for a hardtop on my Jeep. Toyota guys are smarter than Jeep guys so I asked here. Thanks!:thumbsup:
     
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  10. Apr 13, 2019 at 12:24 AM
    #50
    Fuergrissa

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    You don’t understand what you’ve just done. :anonymous:
     
  11. Apr 13, 2019 at 4:22 AM
    #51
    Skydvrr

    Skydvrr Well-Known Member

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    It gone!
     
  12. Apr 13, 2019 at 4:42 AM
    #52
    dweagle79

    dweagle79 Active Member

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    Was this one of those Friday Night Posts but for smart people?
     
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  13. Apr 13, 2019 at 5:14 AM
    #53
    stun gun

    stun gun Squad designated shitposter

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    In that case, no information received, irrelevant.


    Op. Overbuild it and take a nap.
     
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  14. Apr 13, 2019 at 6:11 AM
    #54
    fatboyfirebass

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    Equilibrium is at the beam level situation. I was just trying to show, by means of example, that the short cable carries the higher load. By taking the example to an extreme, I had hoped to help show that.

    Sometimes that technique is useful in double checking your work to see if the calculated answer for the question makes sense or if there is something else that needs considered.
     
  15. Apr 13, 2019 at 6:31 AM
    #55
    fatboyfirebass

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    If the cables were attached to the ceiling first at "some" distance apart yes. When I did the quick math, there were some assumptions. i.e. Massless cables, true frictionless connecting points, no variation in the weight distribution of the beam, the attachment points in the ceiling were exactly 4.89897...feet apart, the cable lengths were exactly as outlined, no stretch in the cables under load, etc, etc ,etc.

    For application, in the question posed by OP, the results showed that he the difference in tension of the 2 cables was so low as to be considered negligible.
    I say follow @stun gun advice and overbuild it.

    Use cable that is at least rated for twice the load, 2 lengths of cable at each connection, Use 1 size bigger than needed eyebolts, and leave a few inches clearance all around your hard top.
     
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  16. Apr 13, 2019 at 6:49 AM
    #56
    Mtn Mike

    Mtn Mike Well-Known Member

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    Excellent thread. Time to go to the lab. Who's going to model this in their garage this weekend?
     
  17. Apr 13, 2019 at 6:54 AM
    #57
    Twitcher

    Twitcher Active Member

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    It doesn’t matter if the cables are attached to the ceiling at 90 degrees or not (even to a sloping ceiling) as long as the cables are plumb. It doesn’t matter if the beam is sloping or not. Gravity loads are completely vertical. If no lateral load is applied (as implied in this case), the beam cannot move laterally. And the tension (load in each cable is identical if the beam load is symmetrical).

    In the case of a tandem crane pick the load can vary because it is almost impossible to keep the cables exactly plumb during the pick.

    I’m a retired structural engineer. This discussion always came up with newly minted engineers concerning sloped bridges or walkways. They don’t slide off their piers because they are sloped.
     
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  18. Apr 13, 2019 at 6:58 AM
    #58
    RocTaco

    RocTaco Habitual line stepper

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    Agreed.

    If OP really wants to know, mock it up and hang your beam from 2 luggage scales at your desired angle and cable lengths. That'll tell you what the difference is if any.
     
  19. Apr 13, 2019 at 7:41 AM
    #59
    takoashihaisen

    takoashihaisen Well-Known Member

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    same.

    the torque at the both side is arm length x mass. The center of mass of the the 50 lb bar is at the center of the bar, which is marked as my artistic STAR. Since the center of the mass is located 2.5 ft from the each side and cables are 90 degrees from the ceiling, the effective arm length (single small vertical lines) from A and B are equal. Therefore the torque at the A and B are the same.

    Points A and B will feel the same force.

    I think, if you imagine the infinite length of 50 lb bar, the difference of 2' and 3 ' will be negligible. Without thinking of math or complicated stuff, you can guess same tension at the both sides.
    diagram.jpg
     
    Last edited: Apr 13, 2019
  20. Apr 13, 2019 at 9:10 AM
    #60
    duckytw

    duckytw Well-Known Member

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    100% the same tension in the cable on both ends.
     

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