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Physics question

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

  1. Apr 12, 2019 at 6:46 PM
    #1
    svdude

    svdude [OP] Well-Known Member

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    Ok, not a third gen subject but neither are a chunk of other posts here. I ask here since I know there's a handful of smart engineers and/or engineer types here.

    I'm getting ready to make a hoist contraption but need some simple physics help to aid in my rigging. Best way to describe by idea is to ask:

    If a beam is suspended by cables mounted to a ceiling (both cables 90* to the ceiling) one cable is 2 ft long and the other cable is 3 feet long. The beam is 5 feet long and weight 50 lbs, cables are attached at the ends of the beam at an even distance from the end of the beam.

    Do both cables support the same weight?

    I haven't put this to paper yet (trying to get my boy to sleep first). However, thinking about the free body diagram for each point on the beam there's a sine and cosine factor for both ends... It makes me think that each cable will have different tensions, right? Or am I over complicating this and of course both cables support 25 pounds each?
     
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  2. Apr 12, 2019 at 6:50 PM
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    Rockefelluh

    Rockefelluh Well-Known Member

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    No they don’t. Think about if you held the cables yourself. You would be holding the shorter cable with more strength than the longer one.

    I need to find some paper to write the force diagrams on. But that’s a good thought experiment.
     
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  3. Apr 12, 2019 at 6:52 PM
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    svdude

    svdude [OP] Well-Known Member

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    That's what I would think but I kinda wanna know for sure for my design. I kinda like to either experiment with scales to know for sure of work the problem on paper and know for sure that way. It's been 2 years since I have taken physics 2 or a statics class so I have forgotten most of the fundamentals that would help me solve this type of problem.
     
  4. Apr 12, 2019 at 6:55 PM
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    Twitcher

    Twitcher Active Member

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    Both cable hangers will have the same load as long as the suspended load on the beam is equal distance from the cable hangers (or in other words if the load is in the middle of the beam).
     
  5. Apr 12, 2019 at 6:56 PM
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    4x4Runner

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    If I recall from my rigging days, the vertical weight should be equal to the weight of the beam since the rigging is at 90* to the ceiling.
     
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  6. Apr 12, 2019 at 6:56 PM
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    svdude

    svdude [OP] Well-Known Member

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    Let's say the 50 lbs is evenly distributed through the beam.
     
  7. Apr 12, 2019 at 6:57 PM
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    svdude

    svdude [OP] Well-Known Member

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    So you're saying that both cables are supporting 25 lbs each since they have the same angle to the ceiling?
     
  8. Apr 12, 2019 at 6:58 PM
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    Twitcher

    Twitcher Active Member

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    Would still be the same tension or load in each cable.
     
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  9. Apr 12, 2019 at 7:02 PM
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    styk

    styk Well-Known Member

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    Is this a trick question? In general yes but...... if the cables are attached to the beam very close to center, then the short cable could end up with all the weight with nothing exerted on the longer cable other then it's own weight. But it probably wouldn't hang straight down in that case.
     
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  10. Apr 12, 2019 at 7:03 PM
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    Jaysonn

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    The shorter cable that has the the beam higher in the air will take more weight/tension then the longer cable. Only reason i can say this is because i do tandem lifts with cranes and the moment i take a load higher then the other crane i pull more weight on my scale and the other crane loses weight.
     
  11. Apr 12, 2019 at 7:04 PM
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    4x4Runner

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    Yes. I if I recall, it’s when you start increasing the angle from the hoist point does it start increasing the strain on the rope or sling.

    If you held a 50lb bar in your hands with your arms straight down and one end attached with a 1’ extension, does it feel heavier?
     
  12. Apr 12, 2019 at 7:06 PM
    #12
    fatboyfirebass

    fatboyfirebass Well-Known Member

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    establish X to the right, and Y up are positive
    Beam angles from lower left to upper right
    the beam is static. no "X" component.
    Figuratively "Cut" the cables. summate "Y" Components; Y1 + Y2 -50 =0
    Assume uniform weight for the beam
    Center of mass is 2.5 feet along the beam from the left end but due to the angle of the beam, this is (square root of 24)/2 from the left end in the x direction.
    Establish that clockwise moment is positive.
    summate moments about the left end of the beam
    {((Square root 24)/2)*50}-{(square root 24)*Y2}=0
    122.47=4.90*Y2
    reaction force Y2=25.0000025 LbF in the positive Y direction
    therefore Y1 = 24.9999975

    Damn near 25 lbs each
     
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  13. Apr 12, 2019 at 7:07 PM
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    arguscanis

    arguscanis Well-Known Member

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    cant wait for the explanation, cause if its linear pull i cant see how increased length decreases the weight. would this mean that if you shorten your winch cable its load weight changes?
    then again im convinced the big ball lands first
     
  14. Apr 12, 2019 at 7:08 PM
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    4x4Runner

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  15. Apr 12, 2019 at 7:15 PM
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    markm0311

    markm0311 ________________

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    That’s a statics/dynamics problem :thumbsup:. Draw it up and I’ll do the math for ya
     
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  16. Apr 12, 2019 at 7:16 PM
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    markm0311

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    Never mind. He already did the statics. Listen to this guy.
     
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  17. Apr 12, 2019 at 7:17 PM
    #17
    RushT

    RushT Well-Known Member

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    I think the issue is the bean is horizontal, the roof anchor pints are at different heights. If the beam is horizontal, cables support the same weight as long as it’s distributed.
     
  18. Apr 12, 2019 at 7:19 PM
    #18
    svdude

    svdude [OP] Well-Known Member

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    Statics is still physics based, just a department of physics really. I did take physics and statics a while back but forgot most of it. Gonna draw up a diagram and dig out my physics book and make up a couple free body diagrams for each cable and replicate what was said above.
     
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  19. Apr 12, 2019 at 7:21 PM
    #19
    markm0311

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    This is the best thing I’ve read all day. Good on ya. The math never lies
     
  20. Apr 12, 2019 at 7:26 PM
    #20
    svdude

    svdude [OP] Well-Known Member

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    Here's a quick drawing of my question...

    Untitled.jpg
     
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