Edexcel IAL Biology 4.9 Core Practical 8 plant fibre tensile strength

Use a justified range, measure diameter to calculate cross-sectional area, add load consistently until failure and repeat before comparing stress or force.

Exam points

Define tensile strength through the stress or force a fibre withstands before breaking.
Control fibre length, diameter, source and environment while varying one factor.
Use diameter and breaking force to calculate or interpret tensile-strength trends.

[Maximum number: 7]

Fibres from the stems of hemp plants are widely used in industry for their great tensile strength.

The relationship between tensile strength and fibre diameter was investigated.
The photograph shows a cross section of a part of a hemp fibre and an eyepiece graticule. The fibre bundle is made up of pith cells, some of which are shown here.

Figure for Question 4.9—Core Practical 8 - tensile strength of plant fibres question 1 — Edexcel A-Level Biology AS

(a)

The diagram shows apparatus that was used in an investigation to find the tensile strength of hemp fibres.

Figure for Question (a) — Edexcel A-Level Biology AS

[ 7 ]

(i)

Describe how you could use this apparatus to make a valid comparison of the tensile strength of fibres with different diameters.

[ 4 ]

(ii)

Describe the relationship between the diameter and the tensile strength of these fibres.

[ 3 ]

An answer including three of the following points :

- the larger the diameter the less tensile strength / negative correlation (1)

- non-linear (1)

- steepest decrease \{between $21 \mu \mathrm{~m}$ and $25 \mu \mathrm{~m} / \mathrm{up}$ to $25 \mu \mathrm{~m}$ (1)

- less steep from $\{25-81 \mu \mathrm{~m} / 81-84 \mu \mathrm{~m}\}$ (1)

Accept reverse

(3)

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4.9—Core Practical 8 - tensile strength of plant fibres question 1

Question (a)

Question (i)

Question (ii)