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Breaking Paper

Introduction
Public summary: 

Stretching paper until it breaks can need a surprising amount of force. Find out how much and why aeroplanes and boats have rounded windows.

Stretch pieces of paper to demonstrate just how strong paper is under tension.
Useful information
Kit List: 

Paper stretching board with newton (force) meter.
Roll of fax paper.
Shielded scalpel and cutting mat.
3 paper stamps (Circle, square and heart shaped).

Packing Away: 

All fits in a large red box.
Newton meter should turn itself off.

Frequency of use: 
2
Explanation
Explanation: 

Things to do
Basic idea: Attach a piece of paper between the clamps (needs clamping very tightly), tighten the nut at the end until the paper breaks, notice the reading on the newton meter just before it does. Then try variations on the paper, such as:

* Different widths (full/half)
* Different lengths (should have minimal effect on results)
* Square holes
* Diamond holes
* Rounded holes
* Full width with small cut; with practise you can actually see the crack propagating away from the cut if you tune the force really carefully.

With the holes you can also ask where the paper will break, you should find that it happens next to a corner or a sharp bend where the paper was cut badly (if scalpel was used). Children of a sensible age can use the shielded scalpel/help do up the clamps, though often they're surprisingly bad at doing them up tightly enough!

What can go wrong?
Aside from the risks in the RA, there are a few ways in which this experiment can not work too well unless you're careful to avoid them:

* Clamps not tight enough, meaning that paper slides rather than breaks - if this happens check that the washers are on the outside of the blocks. The paper can also slide out if the piece you are using is not long enough and only a small part of it is clamped. If the length of the paper is long enough to completely go through both clamps and still have a bit of over hang you will likely avoid this problem
* Paper not straight, so it twists and buckles rather than stretches
* Freak results: In particular when you're dealing with holes, the results aren't always as consistent as they might be, eg a square hole can be stronger than a badly cut circular one!
* Scales decide to reset/turn off mid-experiment. There's not much you can do but release the tension and start again if this happens.

Things to talk about:

* What are the scales measuring? The reading is in kilograms, but really it's measuring the pulling force, you may need to persuade the visitors that a sideways pulling force feels just the same as weight on the bottom + gravity (can talk about stretching a child/tying a large weight to their feet if you like...!) With older/brighter children you can talk about converting to Newtons.
* Why do the clamps have foam attached to them? If you get them to move their hands along the surface of both sides of the clamp you can easily tell that there is much more friction with the foam. If there wasn't any foam the experiment wouldn't work as the paper would just slide out.
* Paper is really weak under compression, and tears easily when sheared, but it is strong under tension. It's made up of lots of long fibres, thing of it as lots of tiny bits of string, which are strong when you pull them but collapse when pushed (can compare this to the suspension bridge if they've already seen that).
* The breaking point of a full width piece of paper is about the same as the weight of a typical child in your audience, at 50kg=500N (usually ok to ask children how much they weigh, don't try this with adults...!) So you could almost dangle them off a cliff supported on a piece of paper!
* When the paper is half the width, there are half as many 'bits of string', so it only takes half the force to break it. It's often useful to get a marker pen and draw on the paper to show how the force can be split across the paper (about 100N in each of 5 sections or something like that...remember to explain what you mean by a Newton if you use this terminology).
* Corners are weak. You can think of all of the force for one of the sections being concentrated at a very small point, which causes it to break there. Can make shapes stronger by rounding the edges...circle perhaps ought to be strongest for given area.
* When can this be useful? Perforations in toilet paper or 'tear off' strips concentrate the force in the paper links so it is easier to tear it.

How is this linked to the real world?

* When you go in an aeroplane what shape are the windows? What might happen if they were completely square? The de Havilland Comet I was the first commercial airliner, built in 1949. It passed all safety testing, and went into commercial use, but there were two catastrophic accidents that were eventually traced back to the square windows.

* In WW2, almost 3000 Liberty ships were built in sections, and fitted together later, as a cheap way of transporting massive amounts of cargo from the US to UK. The problem was that they had square joints, which tended to fracture in heavy seas, with disastrous effects...12 ships broke in half unexpectedly for reasons along these lines (trivia: Constance Tipper of Newnham College was first person to understand why this happened). Use of brittle materials was also partly to blame.

* There are probably other examples along these lines that engineers/materials scientists might know, let us know if you think of any more good ones!

PLEASE REMOVE BATTERIES FROM FORCE METER WHEN PACKING AWAY - the batteries have a tendency to drain between events

Risk Assessment
Date risk assesment last checked: 
Sun, 07/01/2018
Risk assesment checked by: 
J_Tumelty
Date risk assesment double checked: 
Fri, 12/01/2018
Risk assesment double-checked by: 
Josh Garfinkel
Risk Assessment: 
DESCRIPTION Stretching paper until it breaks.
RISKS 1. Risk of being hit by paper clamp when paper breaks.
2. Bolts in paper clamp may be sharp.
3. Finger trap risk from wing nuts during tightening.
4. Experiment contains scissors to be used by children.
5. Shielded scalpel is still sharp beneath the shield, and can cause cuts if the shield is pushed aside.
ACTION TO BE TAKEN TO MINIMISE RISKS 1. Demonstator to ensure that children are away from the clamp the first time the paper is broken. After this children will normally be wary enough, but demonstrator to remain vigilant. (Energy released on breaking is comparatively small, since force is applied by tightening a screw and fax paper does not stretch much before breaking)
2. Top of bolts which will be moving covered in tape to minimise potential damage. Demonstrator to replace tape if required.
3. Unlikely to occur, but be aware of the possibility if kids are doing the tightening. Warn children to be careful if required.
4. Don't give scissors to very young children. Supervise children when they're using scissors.
5. Don't give the shielded scalpel to young children, show children how to use it before letting them handle it. If in doubt, keep it to use yourself.
ACTION TO BE TAKEN IN THE EVENT OF AN ACCIDENT Call a first aider in the event of injury.
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Images
Experiment photos: