Lessons 1–2 · 2 lessons · N. Sh. Turdiyev, K. A. Tursunmetov, A. G. Ganiyev, K. T. Suyarov, J. E. Usarov, A. K. Avliyoqulov. Physics Grade 10, 1st edition. Niso Poligraf Publishing House, Tashkent, 2017
1
Research methods of physics
Textbook: pp. 3–4
GoalKnow the research methods of physics: observation, hypothesis, experiment, model, law and theory.
Physics is an experimental science: phenomena are observed, a guess about a regularity – a hypothesis – is made, and then it is tested by an experiment in specially prepared conditions. To study a complicated phenomenon a model of it is built: only the most important features are kept and secondary ones are dropped (a material point, an ideal gas). An experiment can never remove every outside influence, so its results are used to draw conclusions about an “ideal” case (for example, motion without friction); this is called idealization. If a hypothesis is confirmed in many experiments it becomes a physical law; a set of connected laws forms a scientific theory, for example Newton’s laws form classical mechanics. Every theory has a limit of applicability: classical mechanics fails at speeds close to the speed of light and for very small particles such as electrons. A new theory does not cancel the old one; within the old limits it must agree with it – this is the correspondence principle.
Worked examples
Example: a student notices that heavy and light bodies reach the ground almost together. Hypothesis: without air resistance every body falls with the same acceleration. Test: in a tube with the air pumped out a feather and a metal ball fall together. The hypothesis is confirmed – this is the law of free fall.
Example: classical mechanics is enough to compute the motion of a spacecraft flying at 10 000 m/s (about 30 000 times slower than light), but for protons accelerated to 0.99 of the speed of light in a particle accelerator it gives wrong results. So a theory has a limit of applicability.
Class activity
In groups discuss which experiment could test the hypothesis “everything near the Earth falls down” (only paper, a book and a light ball, in safe conditions allowed by the teacher). Record the result in an “observation – hypothesis – experiment – conclusion” table.
Practice
1
Under what condition does a hypothesis gain the status of a physical law?
When many experiments confirm it and none contradicts it.
2
What does “model” mean in physics? Give two examples.
A simplified version of a phenomenon that keeps only its important features; examples: a material point, an ideal gas.
3
In which two cases does classical mechanics give wrong results?
At speeds close to the speed of light and for the motion of particles of very small mass (for example, electrons).
4
Why must all quantities in a physical theory be measurable in experiments? Explain.
Otherwise the theory’s predictions cannot be checked by experiment, so it would stop being science.