practical 4(new)change

8/2/2019 Practical 4(New)Change

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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?

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