UKC

So im looking at setting up a rope swing from the underside of a bridge. (See photo)
Im thinking about jumping/Stepping) from the stone pillar on the left of the photo. As im starting the arc from higher than then main bridge span do the laws of physics say the top of the arc will be the same height as where i started and then decrease.

Or will the stretch in the rope make it longer and take some energy away thus not getting as high. My question is, Will i hit the main bridge span if im stepping off from higher than the anchor.

Im yet to measure the height of the bridge and work out how close the anchor will be to me to stop me hitting the water.

Photo:
https://dl.dropboxusercontent.com/u/61015588/Rope-swing.jpg

In reply to sbattams:

Might jut be the photo but it looks like you'll just hit the water to me.

But assuming that's not the case, you'll drop vertically until the distance between you and the point the rope is tied to is equal to the rope length, then you'll arc as the distance is constrained to the length of rope between the two points and can not increase or decrease so the only option is to follow an arc of that radius.

In reply to sbattams:
I don't think you'll hit the bridge, I reckon too much energy will get taken out by the rope as the difference in heights is little. Best way to be sure though is test it with a large weight first. 70kg bag of rocks?

In reply to Lord_ash2000:

I thought that was the case. The bridge is higher than it looks a good 15m (I think, yet to be measured) So im hoping for a good 10m swing.

It would be on the wonk though as rather than going the length of the bridge im starting form one side you would swing under the bridge a small amount. I was more concerned about whizzing up into the underside of the bridge on the other side, yet was sure there are laws (physics) against that happening. As long as I don't take a huge jump off the pillar.

In reply to Lord_ash2000:

I think he should take a towel and change of clothes!

In reply to Lusk:

Thanks for the vote of confidence.

Im not planning to get wet, but it will be close. Being wintertime im sure the water is lovely and warm.

Steve

In reply to sbattams:


My first reaction was that that made a lot of sense - nice reasoning.

My second reaction was that at the top of your swing your instantaneous velocity is zero (and hence no radial acceleration required to keep you moving in a circle(ish) ). I think there might well therefore not be any rope stretch at that point. Therefore possibly you will make it as high as your starting point after all. (The rope will stretch as you come to the bottom of your swing, but will then un-stretch as you head upwards again)

On the other hand, if you start with the rope slightly slack (as you inevitably will do) then "common sense" says that you will not end up back as high as you started - although my addled brain is struggling to come up with a simple physical justification of that statement right now...

In reply to sbattams:

I can't see the picture, However I think what you are after is what height will you come up to on the otherside of the swing.

Law of conservation of energy indicates that if you start statically (i.e. don't jump/take a runup) then you will reach the same height on the other side as you started. However in the real world, Friction and drag will remove some of the energy from the system which means you won't achieve the starting height (but there won't be much in it).

However, as anyone who's played on a swing knows, moving your body weight (i.e. putting energy into the system) will result in you going higher, so be careful that you don't automatically try to "swing" as you would on a playground swing.

In reply to balmybaldwin:
beat me to it. last paragraph that is.
nice simple way of considering it here :
http://www.bbc.co.uk/schools/gcsebitesize/science/add_ocr_pre_2011/explaini...

In reply to balmybaldwin:

... but your answer involving conservation of energy does not acknowledge his point about the rope stretching. As I wrote, I don't think it *will* actually be stretched at the limit of the upswing, but it's a very important and relevant question to ask.

In reply to sbattams:

I think you're going to end up wet and very bruised.

In reply to sbattams:

Have you considered hitting the wall on the way back?

In reply to sbattams:

You can't have more gravitational potential energy than you started with so you can't go higher

Energy will be lost due to drag and rope stretching
Once you have fallen and are moving in and arc the tension in the rope will change.
At the bottom of the arc = centripetal force + weight
At side = centripetal force

In reply to sbattams:


No worries!
Don't forget to post a video up.

In reply to sbattams:

F*ck the laws of physics. Murphy is stalking you.

Ask a friend to dial the first two nines just before you jump.

In reply to sbattams:

ask these guys

youtube.com/watch?v=1-HGC0GLwXs&

In reply to sbattams:

Done this a couple of times, it's surprising how little energy is lost with each swing. On most bridge swings you drop a bit before hitting 'the arc' which kills alot of the swing. You will need to have a fairly tight rope before you jump (so you don't hit the pillar you jumped from - but you probably will anyway) so will already be on the arc as you jump. I don't think much energy will be lost to stretch/air resistance and I'm pretty certain you will hit the underside of the bridge. And then hit the pillar you jumped from on the way back.
Your best bet would set it up so you swing under the bridge (like most bridge swings) if possible.
Or wear a helmet, and video it
Hope that makes some sense

In reply to mark20:

Great information, Thanks, think I might change where I launch from to avoid the wall on the return swing.

Steve

In reply to sbattams:


How are you planning to get back up to the bridge once you stop swinging?

Regardless of advice from the armchair physicists I'd test any rope swing by filling a sack with rocks weighing approx. the same as you.

In reply to Le Chevalier Mal Fet:

I was planning on testing with a big bag weighing as much as me (around 60kg).

Getting down isnt a problem as we will have a waiting open canoe to lower into that has been paddled out by another member of the group. so will jump with a grigri tied off onto the harness incase of slippage then once finished untie until only on grigri and lower down on some spare length of rope that has been stowed awayin a bag or something to a waiting canoe.

Steve

In reply to Le Chevalier Mal Fet:


I believe 'theoretical physicist' is the polite term. Not that there aren't plenty of experiments you can do from your armchair.

In reply to sbattams:

Get your nuts straight and warm your neck up before you jump.

In reply to sbattams:

Since you're dropping from significantly above the bottom of the bridge deck (assuming standing start) there's a fair chance you'll hit it on the way back up the other side. There's also a fair chance of hitting your launch point on the way back. Looks like an all round poor choice to me. Watch you don't saw the rope at the 'pivot'.

jk