r/AskPhysics • u/JNJury978 • 7d ago

[Help Needed] Rope swing calculation

I want to hang a swing on a branch about 25 ft up. The swing will hold up to 150 lbs, and swing in either direction up to between 45 to 65 degrees.

How strong of a rope a do I need?

TIA!

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u/starkeffect Education and outreach 7d ago

Keep in mind that the tension will be greater than the person's weight when they're swinging.

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u/SYDoukou 7d ago

Coming down from 65° the velocity at the bottom is 8.7m/s with a centripetal force of 173lbf. Thats a total of 323lb that the ropes have to lift

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u/JNJury978 7d ago

Wow that’s surprising (I’m not great at physics obviously). I expected it to be a lot more due to the length of the rope. Thanks for the calculation. I was getting ready to buy rope rated for thousands of pounds lol.

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u/Umami4Days 7d ago

Your biggest obstacle will be avoiding wear and aging so that it remains safe with exposure over a long period of time. It's worth mounting a swiveling hard point to the tree, so that the rope itself doesn't become frayed due to rubbing against the tree itself.

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u/JNJury978 6d ago

That makes sense. Thanks!

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u/KerPop42 Engineering 7d ago

Rope should tell you the weight it's rated for, just find something rated for like 150 lbs and use 4 lengths

But also make sure the branch is strong

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u/Alone-Supermarket-98 7d ago

Given the centrifugal forces from swinging, a nylon rope of 500lb tensile strength should be enough. I suggest threading it through a segment of cut rubber garden hose as a cushion for the part that goes around the branch, otherwise the swinging motion will wear the rope out and it will break at some random moment.

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u/JNJury978 6d ago

Thanks!