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Core Practical 8

Determine the tensile strength of plant fibres
OBJECTIVES
● To know how to determine the tensile strength of plant fibres
● To be able to comment on experimental design and evaluate scientific methods

SAFETY
● Use a sharp knife, place the celery on the tile and cut down onto the tile.
● Wash hands with soap and water after the practical work is over.
● Take care when using the knife to cut the celery.
MATHS SKILLS
● Translate information between graphical, numerical and algebraic forms.
● Plot two variables from experimental or other data.
EQUIPMENT
● celery (or other suitable plant stems)
● two retort stands with clamps
● knife
● white tile
● 10 g and 50 g masses
● plastic tray with cotton wool or other suitable cushioning

DIAGRAM

figure A Adding masses to the plant fibre string

PROCEDURE
Although the retting process can be used to produce plant fibres, it is time consuming. Therefore, a simpler process of extracting plant fibres will be used in this investigation.
1. Carefully remove nine fibrous strings from the celery stalks, using the knife. Use a white tile as a cutting surface.

2. Carefully inspect the strings to ensure there are no breaks or cuts. Each string should have a constant diameter along its whole length.
3. Cut the strings so you have three that are 10 cm long, three 15 cm long and three 20 cm long.
4. Clamp one of the 10 cm strings between two retort stands. Ensure it is held securely.
5. Place the cushioning underneath the string. Ensure the tray is kept beneath the masses, so they do not strike the bench when the string breaks.
6. Add masses 10 g at a time until the string breaks (see figure A). Record the mass in the results table.
7. Repeat steps 4–6 with each of the other strings.
ANALYSIS OF RESULTS
1. Draw a graph to show the pattern you found in your results.
2. Describe the pattern shown by your results.
LEARNING TIPS
● The plant fibres being used are the xylem tissues. These can be identified by adding coloured dye to the water the celery stem is placed in.
● These fibres are non-living, so they are not easily digested by bacteria in the retting process and are therefore left behind. The retting process is used with different plants when they are processed to produce fibres.