(a) Distinguish between stress and strain as used... | WAEC SSCE - Physics - 2001

Question 1 Report

(a) Distinguish between stress and strain as used in elasticity.

(b) When a force of 40 N is applied to the free end of an elastic cord, an extension of 5cm is produced in the cord. Calculate the work done on the cord.

Answer

(a) Stress and strain

Stress is the force acting per unit cross-sectional area of the material:

\[ \text{stress} = \frac{\text{force}}{\text{area}} \quad (\text{unit: N m}^{-2}\ \text{or Pa}) \]

Strain is the extension produced per unit original length of the material:

\[ \text{strain} = \frac{\text{extension}}{\text{original length}} \quad (\text{no unit, it is a ratio}) \]

Thus stress measures the intensity of the applied load while strain measures the relative deformation it produces.

(b) Work done on the cord

For an elastic cord obeying Hooke's law, the work done equals the elastic potential energy stored, given by the area under the force-extension graph:

\[ W = \tfrac{1}{2}Fe \]

With \(F = 40\,\text{N}\) and \(e = 5\,\text{cm} = 0.05\,\text{m}\):

\[ W = \tfrac{1}{2}(40)(0.05) = 1.0\,\text{J} \]

Read lesson note on Work, Energy And Power (WAEC)

Read lesson note on Elasticity (JAMB)

Answer Details