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To extract 'commercially usefulfibres' from a plant stem and
investigate their tensilestrength.
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You will need:
Stems of mature stinging nettles or other
plant stems
Bucket or bowl Rubber gloves
Paper towels
Eye protection
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Remove the leaves and any flowersfrom stems of mature stinging nettles.
Place the stems in a bowl or bucket of
water so that they are completely
submerged.The stems need to be soaked for at
least a week.
Remove the stems from water.Wash the stems to remove the softened
the tissue and then dry the remaining
fibres
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The outside cuticle and epidermal
layer will rub away and the centralpith will be left when you peel away
the fibres.
These fibres are made up of
vascular tissue; they contain both
xylem vessels and the sclerenchyma.
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Stinging nettleharvesting Cleaning
stem
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Place in a
bucket of water
Taking of
bark
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Bark took off Stinging nettlebark drying
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Stinging nettle string
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Why remove leave and flowers? They form a slimy mass as they rot and make
the smell of the rotting nettles even worse!
Why soak the nettles in water?
To soften the tissue
Both outside and inside the vascular bundles
will wash away in water easier!
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Wear eyeprotection andgloves to protect
the eyesWHY?
To avoid being
stung whenhandling unrettednettles.
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StrengthTensile
StrengthDefined as the
maximum
stress amaterial can
withstand
without failing(breaking)
The maximum
stress caused
by pullingforce that amaterial can
withstandwithout failing.
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The maximum stress caused by a
pushing force that a material
can withstand crushing
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Mature stinging nettle plant stems
Retort stand and clamps
5g weights
Ruler
Scissors
Safety goggles
Mattress
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Independent Variable: Strength of fibre
Dependent variable: Mass of weight required to
break the fibre
Controlled variable :Initial length of fibres,diameter of fibre, samemass of weights,temperature ,humidity,stored for the same lengthof time(as they get olderthey become more brittle),
type/source of fibre
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Different types of fibres have different
strengths
Tensile strength depends on CROSS-
SECTIONAL AREA of fibres Tensile strength of fibre is the MAX load it
can withstand before it breaks
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1) Attach the fibre(Initial length:10cm) to aclamp stand ,hang a weight from the otherend.
2) Keep adding the weights(5g), one at atime, until the fibre breaks.
3) Record the final length of fibre using rulerand calculate the extension.
4) Record the mass needed to break thefibre.
5) Repeat the experiment with differentsamples of the same fibre to increasereliability of result.
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5) The fibres being tested should always
be the SAME length.
6) Throughout the experiment, all other
variables such as temperature, types
of fibres used and length of fibres
must be kept constant.
7) Safety measures must be taken into
account.
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Mass added (g) Initial length(cm)
Final length(cm)
Extension(cm)
5 10
10 10
15 10
20 1025 10
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Questions
What is the structure of cellulose?2
Why is cellulose such a strong molecule?3
Is the string perfectly elastic? Explain your answer4
Does the diameter of the string make a difference?
Extension 1
: Apply Hookes law (F = kx) to this practical to work outthespring constant for cellulose.
Extension 2
: aside from strength, what other properties doescellulose havethat make it a useful molecule to plants?