Holding a Heavy Bag Perfectly Still Is Zero Work Done
Learn why scientists build simplified models, how everyday words like force and work change meaning in science, the difference between a law, a theory and a principle, and how to make a claim testable.
Can you do a lot of work and still do zero work?
Yes — and if that sounds like a trick, it is really a warning about vocabulary.
Stand holding a kg bag of rice at arm's length for ten minutes. Your muscles ache, you are exhausted, and in ordinary speech you have done a great deal of work.
In physics, work means force multiplied by the displacement in the direction of that force:
The bag has not moved, so and therefore joules. Not a little work — none at all.
The physics is not denying that you are tired. It is using the word work to mean something narrower and more precise than the everyday word, and refusing to mix the two. Secondary science is full of such words, and learning which ones have been redefined is as much a part of the subject as any calculation. This page covers the opening chapter of the CBSE Class 9 Science course.
Stand holding a kg bag of rice at arm's length for ten minutes. Your muscles ache, you are exhausted, and in ordinary speech you have done a great deal of work.
In physics, work means force multiplied by the displacement in the direction of that force:
The bag has not moved, so and therefore joules. Not a little work — none at all.
The physics is not denying that you are tired. It is using the word work to mean something narrower and more precise than the everyday word, and refusing to mix the two. Secondary science is full of such words, and learning which ones have been redefined is as much a part of the subject as any calculation. This page covers the opening chapter of the CBSE Class 9 Science course.
Why do scientists deliberately leave things out of a model?
Because a description that included everything would be as complicated as the thing itself, and so of no help at all.
A model is a simplified stand-in for a real system, keeping the features that matter for one particular question and discarding the rest. The skill is not in simplifying — anyone can leave things out — but in choosing which things.
An everyday model you already trust. A metro route map keeps the order of the stations and which lines connect where. It throws away the actual distances, the curves of the track, the depth below ground and the position of every signal. For the question how do I get from here to there? it is close to perfect, and a geographically accurate map would be harder to use.
But ask how long is the tunnel between two stations? and the route map is useless. A model is not right or wrong in itself; it is adequate or inadequate for a particular question.
A worked example from physics. To find how long a cricket ball takes to fall m, the usual model keeps gravity and ignores air resistance. Taking and using :
Drop a real cricket ball and the measured time is very close to second, so ignoring the air was justified.
Now change the object and the same model fails. Drop a feather the same m and it takes many seconds, not one. The model has not become less true — air resistance was always being ignored — but for a light object with a large surface, that ignored effect is now the dominant one.
This is why every scientific statement comes with conditions. Assume the surface is frictionless, assume the gas is ideal, neglect air resistance, assume the wire has no resistance — these phrases are not padding to be skimmed. Each one names something the model has deliberately left out, and each one marks the boundary beyond which the answer stops being trustworthy.
A model is a simplified stand-in for a real system, keeping the features that matter for one particular question and discarding the rest. The skill is not in simplifying — anyone can leave things out — but in choosing which things.
An everyday model you already trust. A metro route map keeps the order of the stations and which lines connect where. It throws away the actual distances, the curves of the track, the depth below ground and the position of every signal. For the question how do I get from here to there? it is close to perfect, and a geographically accurate map would be harder to use.
But ask how long is the tunnel between two stations? and the route map is useless. A model is not right or wrong in itself; it is adequate or inadequate for a particular question.
A worked example from physics. To find how long a cricket ball takes to fall m, the usual model keeps gravity and ignores air resistance. Taking and using :
Drop a real cricket ball and the measured time is very close to second, so ignoring the air was justified.
Now change the object and the same model fails. Drop a feather the same m and it takes many seconds, not one. The model has not become less true — air resistance was always being ignored — but for a light object with a large surface, that ignored effect is now the dominant one.
This is why every scientific statement comes with conditions. Assume the surface is frictionless, assume the gas is ideal, neglect air resistance, assume the wire has no resistance — these phrases are not padding to be skimmed. Each one names something the model has deliberately left out, and each one marks the boundary beyond which the answer stops being trustworthy.
Which everyday words mean something different in science?
Several of the commonest ones, and each has a precise definition with an SI unit attached.
Work. Everyday: effort of any kind. Scientific: , measured in joules (J).
Lift that kg bag through m, taking . The force needed equals the bag's weight, N, so
Now carry the same bag m across a level floor. The force you apply is upward and the motion is horizontal, so the displacement in the direction of the force is zero and the work done by that force is J — however far you walk.
Force. Everyday: strength of character, as in force of personality. Scientific: a push or pull that can change an object's speed, direction or shape, measured in newtons (N).
Mass and weight, which everyday speech treats as the same thing. Mass is the quantity of matter, in kilograms (kg), and does not change. Weight is the gravitational force on that mass, in newtons, and changes with location:
A bathroom scale reading * kg* is reporting mass; the force pressing on it is N. **Asking for the weight in kilograms is the single commonest unit error in secondary science, because weight is a force and forces are not measured in kilograms.
Cell. Everyday: a small room, or a battery. In biology: the basic structural and functional unit of life. In physics: a device converting chemical energy to electrical energy. The same word, three unrelated meanings, and the subject heading tells you which is meant.
Reaction. Everyday: a response or a feeling. In chemistry: a process in which atoms rearrange to form new substances. In physics: the equal and opposite force described by Newton's third law.
The SI base units worth memorising with their symbols:
- Length — metre, m
- Mass — kilogram, kg
- Time — second, s
- Electric current — ampere, A
- Temperature — kelvin, K
- Amount of substance — mole, mol
- Luminous intensity — candela, cd
And some derived units: force in newtons (N), energy in joules (J), power in watts (W), pressure in pascals (Pa), frequency in hertz (Hz).
Unit symbols are case-sensitive and never pluralised.** It is N, not n or Ns; kg, not Kg or kgs. Symbols named after a person take a capital letter (N, J, W, Pa, Hz) while the written-out name does not (newton, joule, watt). These are conventions rather than deep science, and they are marked as strictly as anything else.
Work. Everyday: effort of any kind. Scientific: , measured in joules (J).
Lift that kg bag through m, taking . The force needed equals the bag's weight, N, so
Now carry the same bag m across a level floor. The force you apply is upward and the motion is horizontal, so the displacement in the direction of the force is zero and the work done by that force is J — however far you walk.
Force. Everyday: strength of character, as in force of personality. Scientific: a push or pull that can change an object's speed, direction or shape, measured in newtons (N).
Mass and weight, which everyday speech treats as the same thing. Mass is the quantity of matter, in kilograms (kg), and does not change. Weight is the gravitational force on that mass, in newtons, and changes with location:
A bathroom scale reading * kg* is reporting mass; the force pressing on it is N. **Asking for the weight in kilograms is the single commonest unit error in secondary science, because weight is a force and forces are not measured in kilograms.
Cell. Everyday: a small room, or a battery. In biology: the basic structural and functional unit of life. In physics: a device converting chemical energy to electrical energy. The same word, three unrelated meanings, and the subject heading tells you which is meant.
Reaction. Everyday: a response or a feeling. In chemistry: a process in which atoms rearrange to form new substances. In physics: the equal and opposite force described by Newton's third law.
The SI base units worth memorising with their symbols:
- Length — metre, m
- Mass — kilogram, kg
- Time — second, s
- Electric current — ampere, A
- Temperature — kelvin, K
- Amount of substance — mole, mol
- Luminous intensity — candela, cd
And some derived units: force in newtons (N), energy in joules (J), power in watts (W), pressure in pascals (Pa), frequency in hertz (Hz).
Unit symbols are case-sensitive and never pluralised.** It is N, not n or Ns; kg, not Kg or kgs. Symbols named after a person take a capital letter (N, J, W, Pa, Hz) while the written-out name does not (newton, joule, watt). These are conventions rather than deep science, and they are marked as strictly as anything else.
What is the difference between a scientific law, a theory and a principle?
A law says what happens, a theory explains why, and a principle is a broad rule used as a starting point.
A law states a relationship that observations reliably follow, usually as an equation, and it does not explain itself. Newton's law of gravitation says how strongly two masses attract; it does not say what gravity is. The law of conservation of mass says the total mass in a chemical reaction is unchanged; it does not say why.
A theory is a well-tested explanatory framework that accounts for many laws and observations at once. Atomic theory explains why the law of conservation of mass holds: matter is made of atoms, a reaction only rearranges them, and since none are created or destroyed the total mass cannot change. One theory, explaining a law that had already been established by measurement.
A principle is a broad rule or constraint used as a foundation for reasoning. The principle of conservation of energy — energy can be transformed but not created or destroyed — underlies calculations across the whole of physics. Archimedes' principle and Pascal's principle are used the same way.
In science, a theory is not a guess. This is the misconception worth correcting above all others. In everyday speech it's only a theory means it's just a hunch. In science, a theory is the most strongly supported kind of explanation there is — a structure that has survived repeated testing and accounts for a great many separate observations. A hunch that has not yet been tested is a hypothesis, which is a different word for a different thing.
So the ladder is not law above theory. Laws and theories do different jobs, and neither becomes the other with enough evidence. A law is a described pattern; a theory is the explanation of patterns. A law can go on being used exactly as before while the theory explaining it is replaced by a better one — the measurements do not stop being correct.
A worked example of the three together. The law of conservation of mass predicts that burning g of carbon in g of oxygen gives g of carbon dioxide:
Atomic theory explains it: one carbon atom joins two oxygen atoms, and the same atoms are present before and after. And the principle of conservation of energy tells you that the chemical energy released has gone somewhere — as heat and light — even though no formula in the mass calculation mentions it.
Naming matters less than knowing which job a statement does. When you meet a new statement in physics, chemistry or biology, ask whether it is describing a pattern, explaining one, or setting a constraint on what is possible. That question will organise a great deal of what follows this year.
A law states a relationship that observations reliably follow, usually as an equation, and it does not explain itself. Newton's law of gravitation says how strongly two masses attract; it does not say what gravity is. The law of conservation of mass says the total mass in a chemical reaction is unchanged; it does not say why.
A theory is a well-tested explanatory framework that accounts for many laws and observations at once. Atomic theory explains why the law of conservation of mass holds: matter is made of atoms, a reaction only rearranges them, and since none are created or destroyed the total mass cannot change. One theory, explaining a law that had already been established by measurement.
A principle is a broad rule or constraint used as a foundation for reasoning. The principle of conservation of energy — energy can be transformed but not created or destroyed — underlies calculations across the whole of physics. Archimedes' principle and Pascal's principle are used the same way.
In science, a theory is not a guess. This is the misconception worth correcting above all others. In everyday speech it's only a theory means it's just a hunch. In science, a theory is the most strongly supported kind of explanation there is — a structure that has survived repeated testing and accounts for a great many separate observations. A hunch that has not yet been tested is a hypothesis, which is a different word for a different thing.
So the ladder is not law above theory. Laws and theories do different jobs, and neither becomes the other with enough evidence. A law is a described pattern; a theory is the explanation of patterns. A law can go on being used exactly as before while the theory explaining it is replaced by a better one — the measurements do not stop being correct.
A worked example of the three together. The law of conservation of mass predicts that burning g of carbon in g of oxygen gives g of carbon dioxide:
Atomic theory explains it: one carbon atom joins two oxygen atoms, and the same atoms are present before and after. And the principle of conservation of energy tells you that the chemical energy released has gone somewhere — as heat and light — even though no formula in the mass calculation mentions it.
Naming matters less than knowing which job a statement does. When you meet a new statement in physics, chemistry or biology, ask whether it is describing a pattern, explaining one, or setting a constraint on what is possible. That question will organise a great deal of what follows this year.
How do you turn an everyday claim into a testable prediction?
State exactly what you will measure, what you will change, and what you will keep the same. A claim that cannot fail a measurement is not a scientific claim.
Start with a common claim: cold water boils faster.
As it stands this cannot be tested, because faster and cold are not defined. Reframed as a prediction:
*Two identical vessels each holding mL of water, one starting at and one at , heated on identical burners at the same setting, will reach in times and . The claim predicts .*
Now every part is measurable: volume with a measuring cylinder, temperature with a thermometer, time with a stopwatch.
The variables, named.
- Independent variable (what you change): starting temperature
- Dependent variable (what you measure): time to reach boiling
- Controlled variables (kept identical): volume of water, vessel, burner and its setting, whether a lid is used, the room conditions
Now predict the result from theory. The heat needed is , with for water and mL having a mass of about g.
From :
From :
The cold water needs more energy, so at the same rate of heating it must take longer:
So the prediction is that the cold water takes about longer — and the original claim is false.
Notice what the experiment achieved. It did not merely say no. It said no, and by roughly how much, and the reasoning gives a number the measurement can be checked against. A good test does not just decide a claim; it predicts a quantity.
A claim that cannot be tested at all. Chocolate ice cream is the best flavour. No instrument measures best, no result could contradict it, and two honest people can disagree for ever. It is a genuine statement of preference and it is simply outside science — which is a limit on the method, not a criticism of the claim.
Repeat the measurement. One trial of the boiling experiment could be thrown off by a draught or a misread thermometer. Repeating it several times and comparing the readings is what separates a result from an accident, and it is why laboratory work asks for a table of readings rather than a single number.
Start with a common claim: cold water boils faster.
As it stands this cannot be tested, because faster and cold are not defined. Reframed as a prediction:
*Two identical vessels each holding mL of water, one starting at and one at , heated on identical burners at the same setting, will reach in times and . The claim predicts .*
Now every part is measurable: volume with a measuring cylinder, temperature with a thermometer, time with a stopwatch.
The variables, named.
- Independent variable (what you change): starting temperature
- Dependent variable (what you measure): time to reach boiling
- Controlled variables (kept identical): volume of water, vessel, burner and its setting, whether a lid is used, the room conditions
Now predict the result from theory. The heat needed is , with for water and mL having a mass of about g.
From :
From :
The cold water needs more energy, so at the same rate of heating it must take longer:
So the prediction is that the cold water takes about longer — and the original claim is false.
Notice what the experiment achieved. It did not merely say no. It said no, and by roughly how much, and the reasoning gives a number the measurement can be checked against. A good test does not just decide a claim; it predicts a quantity.
A claim that cannot be tested at all. Chocolate ice cream is the best flavour. No instrument measures best, no result could contradict it, and two honest people can disagree for ever. It is a genuine statement of preference and it is simply outside science — which is a limit on the method, not a criticism of the claim.
Repeat the measurement. One trial of the boiling experiment could be thrown off by a draught or a misread thermometer. Repeating it several times and comparing the readings is what separates a result from an accident, and it is why laboratory work asks for a table of readings rather than a single number.
Exam tip
Exam tip: write the unit, and never call weight kilograms
Every numerical answer needs its unit, and an answer without one is usually marked wrong however good the working is.
Mass is in kilograms, weight is in newtons. A kg student weighs N. Writing *weight kg* is the commonest unit error in the whole subject.
Unit symbols are case-sensitive and never pluralised: N, kg, s — not n, Kgs, secs. Symbols from names take a capital (N, J, W, Pa, Hz); the spelled-out name does not.
For work, use the displacement in the direction of the force. Holding something still, or carrying it horizontally against gravity, is J.
Quote the assumptions a model makes — neglecting air resistance, assuming a frictionless surface. They mark where the answer stops being reliable.
For law, theory and principle: a law describes a pattern, a theory explains patterns, a principle is a broad constraint. A theory is not a guess — an untested guess is a hypothesis.
When asked to design a test, name the independent, dependent and controlled variables explicitly, and say what instrument measures each.
Use with for water, and check whether the question gives mass in grams or kilograms before substituting.
And state which meaning you are using when a word like cell or reaction appears — the subject context decides it.
Mass is in kilograms, weight is in newtons. A kg student weighs N. Writing *weight kg* is the commonest unit error in the whole subject.
Unit symbols are case-sensitive and never pluralised: N, kg, s — not n, Kgs, secs. Symbols from names take a capital (N, J, W, Pa, Hz); the spelled-out name does not.
For work, use the displacement in the direction of the force. Holding something still, or carrying it horizontally against gravity, is J.
Quote the assumptions a model makes — neglecting air resistance, assuming a frictionless surface. They mark where the answer stops being reliable.
For law, theory and principle: a law describes a pattern, a theory explains patterns, a principle is a broad constraint. A theory is not a guess — an untested guess is a hypothesis.
When asked to design a test, name the independent, dependent and controlled variables explicitly, and say what instrument measures each.
Use with for water, and check whether the question gives mass in grams or kilograms before substituting.
And state which meaning you are using when a word like cell or reaction appears — the subject context decides it.
Did you know
The map that was as large as the country
Imagine deciding that a map leaves out too much and setting out to fix it. Add every building, then every tree, then every stone and every crack in the road. Keep going and the map grows until it is the same size as the country it describes.
At that point it is perfectly accurate and completely useless. It cannot be folded, cannot be carried, and answering which way to the station? would mean walking across it — which is exactly what you were trying to avoid.
That imaginary map is the clearest statement of why science simplifies. The usefulness of a description comes precisely from what it leaves out. A model that kept everything would have no advantage over the thing itself.
The same logic runs through every diagram you will draw this year. A ray diagram shows two or three rays out of the countless rays leaving a candle, because two are enough to locate the image. A circuit diagram draws a resistor as a rectangle and says nothing about the colour of the wires. A chemical equation records which atoms join, and not the shape of the flask.
Each is a deliberate discarding, and each is judged by one test: does it answer the question it was drawn for? When a diagram fails, the fix is usually not to add more detail but to notice that the question has changed — the feather and the cricket ball needed different models, not a more crowded one.
At that point it is perfectly accurate and completely useless. It cannot be folded, cannot be carried, and answering which way to the station? would mean walking across it — which is exactly what you were trying to avoid.
That imaginary map is the clearest statement of why science simplifies. The usefulness of a description comes precisely from what it leaves out. A model that kept everything would have no advantage over the thing itself.
The same logic runs through every diagram you will draw this year. A ray diagram shows two or three rays out of the countless rays leaving a candle, because two are enough to locate the image. A circuit diagram draws a resistor as a rectangle and says nothing about the colour of the wires. A chemical equation records which atoms join, and not the shape of the flask.
Each is a deliberate discarding, and each is judged by one test: does it answer the question it was drawn for? When a diagram fails, the fix is usually not to add more detail but to notice that the question has changed — the feather and the cricket ball needed different models, not a more crowded one.
Exam relevance
How does this opening chapter feed into JEE and NEET later?
This chapter looks like an introduction with nothing to calculate, and it is the foundation layer for several later chapters that competitive examinations return to constantly.
SI units and measurement grow directly into the Class 11 Physics chapter Units and Measurements — dimensional analysis, significant figures and error estimation — which is examined in JEE Main. Questions there are often of the find the dimensional formula or which combination has the same dimensions type, and they are unanswerable if the base units and their symbols are not automatic. The habit of writing the unit on every quantity, started here, is what makes dimensional analysis possible at all.
The scientific definition of work becomes the Class 11 chapter Work, Energy and Power, relevant to both JEE Main and NEET Physics. There the formula gains a direction factor, , and the zero-work cases from this page become the standard traps: work done by a centripetal force, by the normal reaction on a horizontal surface, or by gravity on a body moving horizontally. A student who has already accepted that holding a bag still is zero work meets those cases as familiar rather than paradoxical.
Mass against weight is a distinction that keeps being tested, often as an assertion-reason question about what changes on the Moon or inside a lift. It feeds into Laws of Motion and Gravitation in Class 11.
Atomic theory and conservation of mass lead into Some Basic Concepts of Chemistry in Class 11 — the laws of chemical combination and the mole concept — which is common ground for JEE Main and NEET. The reasoning on this page, that a reaction rearranges atoms without creating them, is the whole justification for balancing equations and for stoichiometric calculation.
The word cell splits two ways: the electrochemical cell leads to Current Electricity for JEE, and the biological cell to Cell: The Unit of Life in Class 11 Biology for NEET.
Where board and competitive emphasis differ. A board paper is likely to ask you to state the difference between a law and a theory, or to define work. A competitive paper rarely asks for a definition directly; it builds a numerical or assertion-reason question in which the definition is the hidden step — for instance a work calculation whose answer is zero, which cannot be got right by substituting into a formula without thinking about direction.
The single trap that costs the most marks. Treating theory as a weaker word than law, and ranking them. They answer different questions — a law describes, a theory explains — and a question asking which is more certain is testing whether you know that the comparison is not meaningful.
SI units and measurement grow directly into the Class 11 Physics chapter Units and Measurements — dimensional analysis, significant figures and error estimation — which is examined in JEE Main. Questions there are often of the find the dimensional formula or which combination has the same dimensions type, and they are unanswerable if the base units and their symbols are not automatic. The habit of writing the unit on every quantity, started here, is what makes dimensional analysis possible at all.
The scientific definition of work becomes the Class 11 chapter Work, Energy and Power, relevant to both JEE Main and NEET Physics. There the formula gains a direction factor, , and the zero-work cases from this page become the standard traps: work done by a centripetal force, by the normal reaction on a horizontal surface, or by gravity on a body moving horizontally. A student who has already accepted that holding a bag still is zero work meets those cases as familiar rather than paradoxical.
Mass against weight is a distinction that keeps being tested, often as an assertion-reason question about what changes on the Moon or inside a lift. It feeds into Laws of Motion and Gravitation in Class 11.
Atomic theory and conservation of mass lead into Some Basic Concepts of Chemistry in Class 11 — the laws of chemical combination and the mole concept — which is common ground for JEE Main and NEET. The reasoning on this page, that a reaction rearranges atoms without creating them, is the whole justification for balancing equations and for stoichiometric calculation.
The word cell splits two ways: the electrochemical cell leads to Current Electricity for JEE, and the biological cell to Cell: The Unit of Life in Class 11 Biology for NEET.
Where board and competitive emphasis differ. A board paper is likely to ask you to state the difference between a law and a theory, or to define work. A competitive paper rarely asks for a definition directly; it builds a numerical or assertion-reason question in which the definition is the hidden step — for instance a work calculation whose answer is zero, which cannot be got right by substituting into a formula without thinking about direction.
The single trap that costs the most marks. Treating theory as a weaker word than law, and ranking them. They answer different questions — a law describes, a theory explains — and a question asking which is more certain is testing whether you know that the comparison is not meaningful.
Key takeaways
Models, scientific words and testable claims: quick revision
- A model keeps the features that matter for one question and discards the rest. A metro route map is excellent for planning a journey and useless for tunnel lengths — models are adequate or inadequate, not right or wrong.
- Ignoring air resistance, a m fall takes s from , which matches a cricket ball. The same model fails for a feather, where the ignored effect dominates.
- Phrases such as neglect air resistance or assume a frictionless surface name what has been left out and mark the limits of the answer.
- Work in science is in joules, using displacement in the direction of the force. Lifting kg through m is J; holding it still, or carrying it horizontally, is ** J.
- Force is a push or pull in newtons, not strength of character.
- Mass is matter in kilograms and does not change; weight** is in newtons and does. A kg student weighs N. Never give a weight in kilograms.
- Cell means a unit of life in biology and an electrical source in physics; reaction means atom rearrangement in chemistry and an opposing force in physics.
- SI base units: metre m, kilogram kg, second s, ampere A, kelvin K, mole mol, candela cd. Derived: newton N, joule J, watt W, pascal Pa, hertz Hz.
- Symbols are case-sensitive and never pluralised — N, not Ns.
- A law describes a pattern, usually as an equation, without explaining it. A theory is a well-tested explanation accounting for many laws. A principle is a broad constraint used as a starting point.
- A theory is not a guess. An untested guess is a hypothesis, and a law does not become a theory with more evidence — they do different jobs.
- Worked together: the law of conservation of mass gives g for carbon burning in oxygen; atomic theory explains why; the principle of conservation of energy accounts for the heat released.
- A testable prediction names the independent, dependent and controlled variables and the instrument for each. Cold water boils faster becomes a comparison of mL at and at on identical burners.
- gives kJ against kJ, a ratio of about — so cold water takes roughly longer, and the claim is false.
- Chocolate ice cream is the best flavour is not testable, because best cannot be measured.
- Repeat measurements — one reading can be an accident.
Take three claims you have heard repeated and try rewriting each as a prediction with a named measurement — the ones that resist rewriting are the ones worth being sceptical about.
- Ignoring air resistance, a m fall takes s from , which matches a cricket ball. The same model fails for a feather, where the ignored effect dominates.
- Phrases such as neglect air resistance or assume a frictionless surface name what has been left out and mark the limits of the answer.
- Work in science is in joules, using displacement in the direction of the force. Lifting kg through m is J; holding it still, or carrying it horizontally, is ** J.
- Force is a push or pull in newtons, not strength of character.
- Mass is matter in kilograms and does not change; weight** is in newtons and does. A kg student weighs N. Never give a weight in kilograms.
- Cell means a unit of life in biology and an electrical source in physics; reaction means atom rearrangement in chemistry and an opposing force in physics.
- SI base units: metre m, kilogram kg, second s, ampere A, kelvin K, mole mol, candela cd. Derived: newton N, joule J, watt W, pascal Pa, hertz Hz.
- Symbols are case-sensitive and never pluralised — N, not Ns.
- A law describes a pattern, usually as an equation, without explaining it. A theory is a well-tested explanation accounting for many laws. A principle is a broad constraint used as a starting point.
- A theory is not a guess. An untested guess is a hypothesis, and a law does not become a theory with more evidence — they do different jobs.
- Worked together: the law of conservation of mass gives g for carbon burning in oxygen; atomic theory explains why; the principle of conservation of energy accounts for the heat released.
- A testable prediction names the independent, dependent and controlled variables and the instrument for each. Cold water boils faster becomes a comparison of mL at and at on identical burners.
- gives kJ against kJ, a ratio of about — so cold water takes roughly longer, and the claim is false.
- Chocolate ice cream is the best flavour is not testable, because best cannot be measured.
- Repeat measurements — one reading can be an accident.
Take three claims you have heard repeated and try rewriting each as a prediction with a named measurement — the ones that resist rewriting are the ones worth being sceptical about.