# Rope Swing, starting in tension vs slack

• kgehrels
kgehrels
TL;DR Summary
Is there a difference in the force applied to the pivot point of a pendulum if the rope is slack or tight at the start.
I'm the technical director for a theatre group and am looking at the best way to get an actor to use a rope to swing on stage. As we rent the theatre we use I need to run everything by their staff, who doesn't have a solid grasp of mechanics and physics. I proposed that we lower a pipe from the ceiling, hold it in place with a V of ropes on each end and fasten the swinging rope to this pipe. The actor would start off stage, on a platform that is 1m high and swing onto the stage. The pivot point would be somewhere in front of where the actor starts the swing.

This individual is stating that there is a difference between the actor pulling the rope tight then swinging and jumping off the platform with the rope slack.

Something about this statement doesn't ring true to me. The way I figure it, as tension is applied to the rope the actor will start to swing onto the stage. While there will may be a slight increase in acceleration due an increase in elevation due to the jump that the actor would have to do to get off the platform it would be negligible (I'm figuring a 1 foot jump or so, not the maximum vertical that can be achieved). Thus, other than a slight increase in force at the bottom of the swing, the pivot point will not experience a difference between the two methods of starting the swing. It certainly won't experience the significant amount of force that would happen if the pivot point was above the platform and the actor jumped off the platform, straight down.

I've been researching the topic online but every example that I can find deals with the rope being under tension at all times and doesn't address the situation of the rope starting in a slack state. To me this says one of two things, either I'm right and it doesn't matter or it's a very niece topic and I'm not using the correct search terms. I've been wrong before and don't have the strongest grasp of physics myself so I would like to double check if my intuition is correct or if this individual is correct.

Welcome to PF.

Here are a couple thoughts:

** There is no good reason that I can see to try to "jump" at the start of the swing. That just increases the stresses on the rope and on the actor's hands/wrists/arms. The safest and smoothest swing would start with the rope taut.

** Have you discussed this rigging with your stage hands and your stage company's insurance company? There are probably some special anchor points in the fly loft that are meant for carrying weight and transient loads.

sophiecentaur
I agree that there is no good reason to start a swing with a jump. I also think that human nature would be to pull the rope tight before swinging. However, those arguments hold no weight with this person because there is a small chance that someone can do something unexpected.
Our stage hands are volunteer and the insurance is basic liability that is not geared towards theatre because we are a community theatre group. I'm hiring an engineer to look over the plans from a structural perspective. I posed the question to this group not to get advice for a safe way to rig the rope but for input on the physics of starting a swing with a rope under tension vs a rope that is slack.

kgehrels said:
I'm hiring an engineer to look over the plans from a structural perspective.
Great, that is a good move. This thread is done.

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