Concept, Law, Principle, Postulate, Hypothesis and Theory.
Ohm’s law, Einstein’s theory, Avogadro’s hypothesis. All three carry a scientist’s name, and they are not the same kind of thing. Trust the name and you slip in the exam. Trust the definition and you do not.
1. Concept, law and principle
A correct understanding of something is a clear concept of it. Put another way, a concept is an idea, a line of thought or a general opinion. Take the concept of heat: heat is a form of energy. Applied to a body it warms the body and raises its temperature, and removing it lowers the temperature.
When a theory has been proved by many experiments and its main points can be put in a single statement, it is called a scientific law.
A law gets its name in three ways. From its discoverer: Ohm’s law, Boyle’s law. From its subject: the law of conservation of energy, the laws of thermodynamics. From both: Newton’s laws of motion, Galileo’s law of falling bodies.
A natural truth that can be proved directly and clearly, and with whose help many natural phenomena can be proved, is a principle. Doppler’s principle and Heisenberg’s uncertainty principle are examples.
2. Postulate and hypothesis
When some preconditions are accepted in order to set up a mathematical model or a law, those conditions are called postulates. A scientific theory usually starts with a general statement, and that statement is the postulate. Niels Bohr adopted two postulates for his atomic model. Einstein’s special theory of relativity also rests on two fundamental postulates.
To explain the cause of something they have observed, scientists make assumptions that fit the natural laws already known. These assumptions are called hypotheses.
A hypothesis gives a first explanation of an event. To check it, we run experiments, and the result may support it or reject it. One proved true turns into a theory. Even so, some hypotheses keep the name after being proved, Avogadro’s hypothesis for one.
3. Theory, and “all … but not all”
A theory is built from hypotheses and laws: a hypothesis proved by experiment is a theory. And when a theory can be explained through concepts, statements and equations, that theory is called a law.
This gives the board’s favourite sentence: all laws are theories, but not all theories are laws. Likewise all theories are hypotheses, but not all hypotheses are theories.
A theory is also named after its discoverer or its subject: Einstein’s theory of relativity, quantum theory.
Call each of the six sentences true or false yourself, then tap the row to check.
gridwhich sits inside which
True or false?
sentence
answer
All laws are theories
?
All theories are laws
?
All theories are hypotheses
?
All hypotheses are theories
?
A hypothesis proved by experiment is a theory
?
A postulate comes after the theory
?
True. A theory that can be explained through concepts, statements and equations is what we call a law, so every law is a theory.
False. Only a theory that can be explained through concepts, statements and equations is called a law. Not every theory is.
True. A theory is a hypothesis proved by experiment, so every theory began as a hypothesis.
False. An experiment may reject a hypothesis. Only a proved one becomes a theory.
True. This is the book’s own definition: a hypothesis proved by experiment is called a theory.
False. A scientific theory usually starts with a general statement, and that is the postulate. The postulate comes first.
4. Know it by what it does
The book’s exercise and summary say this: a formal line of thought put forward to explain something is a hypothesis. Now sort ten examples into six boxes. Go by what each one does, not by the word “theory” in Einstein’s or “law” in Boyle’s.
sort thempick by what it does
Which word?
Pick up a chip, then drop it in a box. Trust the definition, not the name.
Right. Next one.
Not yet. Forget the name and ask what it does: a guess to explain, a condition accepted, a proved truth, or a single statement?
All in place. Heat: the concept of an energy that warms a body. Ohm’s and Boyle’s are laws because many experiments proved them. Doppler and Heisenberg are principles, truths that can be proved directly. Bohr’s and Einstein’s conditions are postulates, preconditions accepted to build a model or theory. Avogadro’s stays a hypothesis even after proof. Relativity and quantum are theories by name.
5. How a theory is built
Climb the ladder yourself first. Swing a pendulum, time it, make a guess, then test whether the guess survives.
build a theorytime it, then guess
From a pendulum to a law
The periods are a model, not your measurements.
A theory is not just a guess. It runs from observation through hypothesis and experiment to theory, and last to law: five steps, arranged from the book’s paragraphs on hypothesis and theory. Tap them in order.
in orderthe event first, the equation last
Put the steps in order
What happens before anything else? Something has to catch the eye.
Yes. Then?
Not yet. Ask what this step needs before it can happen.
Right order. Observation comes first, since with nothing seen there is nothing to say. Then the hypothesis, because explaining needs a guess first. Then the experiment, because without a hypothesis what would be tested? Only once proved by experiment is it a theory. And a theory stated in concepts, statements and equations is a law.
Exams ask for the definitions word for word. On each of the six cards, say it yourself first, then turn it over.
term versus definitionsay it first, then flip
One definition each
TermConcept
DefinitionA correct understanding or clear sense of something; an idea or a general opinion.
TermLaw
DefinitionA theory proved by many experiments, with its main points expressed in a single statement.
TermPrinciple
DefinitionA natural truth provable directly and clearly, which proves many phenomena. For example Doppler’s principle.
TermPostulate
DefinitionA precondition accepted to establish a model or law. A theory usually starts with it.
TermHypothesis
DefinitionAn assumption, consistent with known natural laws, made to explain the cause of an observed event.
TermTheory
DefinitionBuilt from hypotheses and laws; a hypothesis proved by experiment is a theory.
6. Check yourself
The first question is on a definition.
check yourselfpick one answer
Question 1
What is a hypothesis proved by experiment called?
A principle is a natural truth provable directly; it is not a hypothesis proved by experiment.
A postulate is a condition accepted without proof, not a hypothesis that experiment has proved.
A law comes one step after a theory, when its main points are put in a single statement.
Right. By the book, a hypothesis proved by experiment is called a theory.
In the second, the book’s definition has the last word.
check yourselfpick one answer
Question 2
Which one is still called a hypothesis even after being proved?
Right. Avogadro's hypothesis kept the name hypothesis even after being proved.
Relativity is called a theory in the book; its name does not carry the word hypothesis.
Ohm's law is a law named after its discoverer, not a hypothesis.
Doppler's principle goes by the name principle, not hypothesis.
In the last, think about which sits inside which.
check yourselfpick one answer
Question 3
What is a statement or guess that has not yet been proved called?
A theory forms once hypotheses are supported by experiment.
Right. A statement not yet proved is called a hypothesis.
A concept has no specific meaning of the kind a hypothesis has.
A principle is used for established truths.
The whole note on seven cards.
cut from the notetap to turn over
Seven cards
QuestionWhat is a scientific law?
AnswerWhen a theory is proved by many experiments and its main points are stated in a single statement, it is a scientific law.
QuestionWhat is a principle?
AnswerNatural truths that can be proved directly and clearly, and with whose help many natural phenomena can be proved.
QuestionWhat is a postulate?
AnswerPreconditions accepted in order to establish a mathematical model or law.
QuestionWhat is a hypothesis?
AnswerThe assumptions scientists make, consistent with known natural laws, to explain the cause of an observed event.
QuestionWhat is a hypothesis proved by experiment called?
AnswerA theory.
QuestionWhy is it true that all laws are theories but not all theories are laws, and not all hypotheses are theories?
AnswerA law is a theory proved by many experiments and stated as equations; a theory is a proved hypothesis; a hypothesis may also be rejected.
QuestionAccording to the book's exercise answer, what is a formal line of thought put forward to explain something?
AnswerA hypothesis.
Source: Physics 1st Paper, Classes 11-12 (NCTB), Chapter 1 “Physical World and Measurement”, section 1.3.