Core practical 8: tensile strength of plant fibres

Method

1. Extract fibres from a plant stem (for example celery, nettle or flax), for example by soaking and peeling, so they are similar in length and thickness.

2. Clamp one end of a fibre firmly to a stand.

3. Hang a mass holder from the other end and add masses (for example 10 g at a time) until the fibre breaks.

4. Record the mass that broke it. Repeat with several fibres and calculate a mean.

Variables and safety

• Independent: type of plant fibre (or age, or treatment).
• Dependent: mass (or force) needed to break the fibre.
• Control: length and diameter of fibre, how it is extracted, moisture content, rate of adding masses.

Safety: wear eye protection (a breaking fibre can flick), keep feet clear and put a tray or soft material under the masses.

Calculating tensile strength

Breaking force (N) = mass (kg) x g, where g = 9.81 N kg-1.

Tensile strength = breaking force / cross-sectional area of the fibre, so thicker fibres need more force to break but are not necessarily stronger per unit area. Measuring diameter (with a micrometer or graticule) lets you compare fibres fairly.

Worked example

A fibre breaks when 1.2 kg is hung from it. Calculate the breaking force (g = 9.81 N kg-1).

1. Force = mass x g = 1.2 x 9.81

Answer: 11.8 N (to 3 significant figures)

Check yourself

1. A fibre breaks with a mass of 2.5 kg. Calculate the breaking force in N (g = 9.81 N kg-1), to 1 decimal place.
   Show answer

   24.5 N. 2.5 x 9.81 = 24.525, so 24.5 N.

2. A fibre with cross-sectional area 0.40 mm2 breaks under a force of 12 N. Calculate its tensile strength in N mm-2.
   Show answer

   30 N mm-2. 12 / 0.40 = 30 N mm-2.

3. A fibre breaks when masses totalling 350 g are added. What is the breaking force in N (g = 9.81 N kg-1)? Give 2 decimal places.
   Show answer

   3.43 N. 0.350 kg x 9.81 = 3.43 N.

4. Which variable should be controlled when comparing fibres from two plants?
   • Length and thickness of the fibres
   • Type of plant
   • Mass needed to break the fibre
   • Breaking force
   Show answer

   Length and thickness of the fibres. Thickness strongly affects the force needed.

5. Which is a sensible safety precaution?
   • Stand directly under the masses
   • Use the heaviest mass first
   • Place a tray under the masses and keep feet clear
   • Remove eye protection to see better
   Show answer

   Place a tray under the masses and keep feet clear. Falling masses can injure feet.

6. Why are sclerenchyma fibres strong?
   • Their walls have many cellulose microfibrils and lignin
   • They contain chloroplasts
   • They are living cells full of water
   • They have thin walls
   Show answer

   Their walls have many cellulose microfibrils and lignin. Secondary thickening with cellulose and lignin gives strength.

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Sources

• Pearson Edexcel International Advanced Level in Biology specification (XBI11/YBI11, first teaching 2018)

Last updated 7 November 2026. Spotted a mistake? Tell us.