A materials science student investigates the elastic properties of a polymer strap.
This is their method:
The table shows the student's results:
| Load (N) | Extension (mm) - Trial 1 | Extension (mm) - Trial 2 | Extension (mm) - Trial 3 | Mean Extension (mm) |
|---|---|---|---|---|
| 15.0 | 4.3 | 4.1 | 4.2 | |
| 30.0 | 8.3 | 8.6 | 8.3 | 8.4 |
| 45.0 | 12.5 | 12.9 | 12.4 | 12.6 |
| 60.0 | 16.6 | 17.0 | 16.8 | 16.8 |
| 75.0 | 21.1 | 20.9 | 21.0 | 21.0 |
(i) Calculate the missing value of the mean extension for a load of 15.0 N to a suitable degree of accuracy.
(ii) A graph of Load (on the vertical axis, in N\text{N}N) against Mean Extension (on the horizontal axis, in mm\text{mm}mm) is plotted. A straight line of best fit is drawn through the data. The line passes through the origin (0,0)(0, 0)(0,0) and the point (21.0,75.0)(21.0, 75.0)(21.0,75.0). Calculate the gradient of this line of best fit.
(iii) Use your answer to (a)(ii) to state the stiffness (elastic constant) of the polymer strap, including the unit.
When the student repeats the experiment, they use the same method, the same equipment, and carry out the trials themselves.
The student says the results show that the experiment is reproducible.
Suggest two reasons why the student is incorrect.
Identify one possible hazard for this experiment and the precaution the student should take.
39 exam-style questions on OCR GCSE Physics Forces in action. Each one has a worked solution and a mark scheme showing where the marks go.