https://theultimatehang.com/hammock-hang-calculator/
Playing around with that if you change the hang angle to 5 then the shear force ramps up to over 1000 pounds. Can some one explain (if there is a video out there even better) how this is possible?
Does this mean there is 1000 pounds of force pulling horizontally? If I had an inline scale and put it between the tree strap and the suspension would it read 1000 pounds?
I'm a structural engineer and know the answer although I think calling it shear is a bit of a misnomer. The tension in your straps is made up of two perpendicular parts which you cant see (a little hard to visualize). Its a right triangle of forces that balance out. The first acts up and down (vertical force) and the second acts in a horizontal direction (horizontal force) and pulls in on the trees. When you reduce the hang angle to 5 degrees the horizontal force gets huge quickly. The vertical force always remains about half of your weight. Its ABOUT half because you probably aren't right in the middle of your hammock and probably aren't exactly half way between the trees. The tension in the strap (called the resultant force) is the square root of the sum of the squares of the vertical and horizontal forces (Pythagorean formula). If you put a scale on the strap it would be measuring the resultant force.
Shear is a bit of a symantic error on that diagram.Originally Posted by onesojourner
In engineering, shear is the force between two layers sliding past each other opposing lateral motion.
The diagram SHOULD say the "horizontal tension component".
The vertical component is what holds your weight up, but because the support lines form the long side of a right triangle, there must be a horizontal component as well.
As you tighten the angle towards horizontal, you still have the same vertical side but the long side of the force triangle must get a lot longer to make that same vertical force for your weight because the vertical side is a function of the sin of the angle. This makes the horizontal side of the force triangle and thus the tension force a LOT longer especially at very low angles as the sin function approaches 0.
Vector Pull should bring up some interesting Google results.
Sorry I'm on my phone but can clarify more if I need to.
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If you are having trouble visualising this, take a dumbbell (or anything heavy) and hold it down by your side. It's now at 90° from the horizontal, and you can hold it just fine. Now raise it up, with your arm outstretched. As you raise it towards 0° (horizontal) your feel it seems heavier. It's the same for your hammock.
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A functional description of the end perception, but not the same effect. The weight held out seems heavier held out by yourself because you're supporting it with contracted smaller muscles instead of almost entirely skeletal structure with much larger core muscles. You're opposing a torque moment in your shoulder, not a tension force.
A better physical analogy would be if you and your buddy are carrying a cooler loaded with ice and your favorite beverage, one on each end.
If your arms are straight down, you are only splitting the weight force of the load. Then, assuming none of the favorite beverage is consumed, you and your buddy are approaching a tree between you, and one decides to go right and one left, starting a sort of "tug of war".
Think of your arms as the hammock straps, and as you open up the angle of your arm towards horizontal, not only are your arms now carrying the weight of the cooler, but you're also having to opposed the pull of your buddy on the other side, thus the total load born in your arm is greater. The weight of the cooler hasn't changed, but the force in your arms increases rapidly as you and your buddy move further apart.
And yes, if you put a scale inline with the suspension, it would read the resultant force of the vertical load and the horizontal tension component (root of the sum of the squares, as has been pointed out). This would be the number reported as the "shear" in the diagram you linked.
"All alone in the night, I had my own thoughts..." ~Kerouac, On the Road
"Help me mamma, for I have grinned..."
Is this creating a mechanical advantage like this?
Shear force, actually the horizontal component of the resultant force (the tree strap), is the force pulling horizontally on the tree. The more horizontal you put a tree strap (<30°), the more horizontal force is pulling on the tree. The more you hang the tree strap towards the vertical (>30°), the more the tree strap pulls downward parallel with the tree (e.g. Has less horizontal component). A tree can handle the vertical component much easier since that is with its grain and its strength. The horizontal component is a lot more abusive to a tree, damaging that delicate cambium layer. This is why 30° is "optimal". It balances out the horizontal and vertical components.
And for the visual folks....
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Derek Hansen over at The Ultimate Hang did this. Can't find the video of it on his website right now, but it's on Youtube.
Would probably be best if we abandoned the use of the term "shear" to describe that tension force component completely, as it is incorrect.
main-qimg-a34d1bc69663c71b5947c5f083e78fad.gif
"All alone in the night, I had my own thoughts..." ~Kerouac, On the Road
"Help me mamma, for I have grinned..."
No, that's a pulley "block and tackle" advantage system that's exchanging Force for Distance in the Work In = Work Out (F*d=F*d) equation.
The "force components" mechanical advantage was called a "vector pull" in my swiftwater rescue courses, but I haven't heard that term used elsewhere.
Here's an excellent walk through of the math, where the hanging mass would be the hammocker and the support lines the suspension ropes:
https://www.ropebook.com/information/vector-forces/
And here's a picture of the mechanical advantage of a "vector pull" in action. Also heard it called "the boyscout method"
Vector-pull.jpg
"All alone in the night, I had my own thoughts..." ~Kerouac, On the Road
"Help me mamma, for I have grinned..."
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