Scientific method. Science is a kind of organized and testable knowledge, which let scientists give a reliable prediction about a subject

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1 Scientific method 1.- What is science? 2.- The scientific method 3.- The scientific report 4.- SI of units 5.- Error and accuracy 1.- What is science? Science is a kind of organized and testable knowledge, which let scientists give a reliable prediction about a subject Activity 1: Can you give several examples of different sciences? Activity 2: Do you find that History is a Science? Do you trust in the information of History books? Generally speaking, there are three types of Science: Experimental, Social and Formal. Experimental sciences are focused on the study of the Universe. Its name comes from the fact that predictions are usually tested by means of experiments carried out in laboratories. Some examples of experimental sciences are Physics, Chemistry, Biology, Meteorology, etc. Social sciences study different phenomena related to the Humanity, such as Geography, History, or Psychology. Their predictions are also tested by different ways because socials scientists cannot perform experiments as experimental scientists do. Formal science is knowledge about abstract or non-real objects, such as numbers. Mathematics, Logic or Computer sciences are examples of Formal sciences. 2.- The scientific method The scientific method is a procedure, which is used by all the scientists since Galileo Galilei developed it in the XVII th century for the first time. Generally speaking, it has several steps, but sometimes some of them could not take place to the same degree A) Observation and asking a question Although some people have a great knowledge of nature, we cannot consider them as scientists because scientific and common observations of a physical phenomenon are quite different. Activity 3: Can a farmer give you an accurate weather forecast for his town? Do you think he is a meteorologist? Activity 4: Search information about the discovery of radioactivity by Henri Becquerel in 1895 An observation of a physical phenomenon needs some features to be considered as a scientific one. It must be:

2 Systematic: it must be carried out periodically Exhaustive: it must be performed completely, by all the possible ways Accurate: the observation must be as much accurate as possible Reproducible: the procedure must be explained to let everybody repeat the experiment The main purpose of scientific observation is raising a question related to Nature. As a matter of fact, every scientific research is an attempt to give an answer to a certain question. There are different kinds of questions: Why is the rainbow formed? Do different bodies fall down to the ground at the same rate? Why do Spanish children become fatter? B) Formulation of a hypothesis A hypothesis is a conjecture, a statement or an explanation based on our previous knowledge of the Universe, which can explain some phenomena and be tested empirically. You can formulate different hypothesis to explain a question. Here are some examples: The rainbow appears when light passes through water drops Heavy bodies fall faster than lighter ones Spanish children become fatter because of crisps C) Experimentation A scientific hypothesis must be stated in terms of measurable quantities to let us check whether it is true or false. In other words, a hypothesis must give us a testable prediction so that we can know whether it is right or false after collecting the results of the experiment. Activity 5: Imagine different physical quantities, which can affect the motion of an object: mass, size, temperature. How can you test its impact on the motion? A variable is a physical quantity, which is measured through an experiment. There are two different variables: independent and dependent variables. An independent variable is a physical quantity that can be controlled through the experiment. On the other hand, the dependent variable is the outcome of the experiment. Experimentation goal is testing separately each variable in order to control them one by one. Galileo wanted to know why objects fall down to the ground so he designed different experiments to test which variable determines its motion. When he tested different masses, he realised that objects of different masses fall down at the same rate. In this experiment the independent variable is mass and the dependent variable is time Activity 6: Imagine you want to know why Spanish children become fatter. The independent variables are the factors that affect the children weight and the dependent variable is the proper weight. How can you design an experiment?

3 D) Statement of a laws and theories A scientific law is a statement, which is verified empirically and has a universal applicability. Activity 7: Try to state the following scientific laws from Physics, Chemistry, Biology: Mendel Laws, Hooke Law, mass conservation Law Activity 8: Do you know Murphy s law? Do you think it is a good example of scientific law? A theory is a set of basic principles, which can be used to deduce different scientific laws without any apparent relationship between them. Usually a theory is more abstract than a scientific law and cannot be tested directly by means of an experiment, although its consequences or predictions must be true. A well-known theory from Physics is the Universal Gravitation Theory, which states that any object attracts another one and the attraction force is proportional to both masses and inversely proportional to the distance squared. This theory can explain: The free fall law of the bodies The motion of planets around the Sun The tidal laws The shape of planets and galaxies Activity 9: Do you know the following theories from Physics, Biology and Chemistry: Relativity theory, Evolution theory, Atomic-molecular theory, continental drift theory, and Cell theory? A model is a representation of a system, which can help us to understand its elements, structure or working-out. Imagine an underground map: it is not exactly the underground network, but we use it to know the stations, correspondences, etc. Activity 10: Try to explain the following models: atomic models, Heliocentric model, heart as a pump and blood vessels as pipes E) Formulation of new hypothesis A theory let scientists predict new hypothesis, which can be tested and prove whether this knowledge is reliable or not. If the results of the experiment are according to the predictions, the theory is confirmed. Otherwise, if some of the new hypothesis is supposed to be false, scientists must create new theories, which can explain the new experimental facts. Activity 11: Ancient Greek scientists believe the Earth was the centre of the Universe, but Copernicus and Galileo disagreed with them. Do you know the experimental evidence which support the new theory? Experiment 1: Study of a pendulum using the scientific method

4 3.- The scientific report Every scientific report, even your lab report, has to be written following some standard conventions to be accepted by the scientists community. We can usually find these elements: a. Tittle The tittle must be clear and meaningful. The scientific report always has an abstract following the tittle, which sums up the results and conclusions of the report b. Introduction The introduction gives the information needed to understand the experiment. It can also be used to explain the goals of the research c. Methodology The methodology has to do with the procedure used through the investigation. Every step must be explained to let other scientists repeat the experiments. Besides you have to include the material used in the experiment d. Results Data must be organized in tables and graphs to show the results of the experiment easily. Data of the same kind are placed in columns with the physical quantity and its unit at the heading of each one. e. Conclusions The conclusions of the research must be clear and respect strictly the experimental evidence. f. Bibliography The written sources used in the research must be listed alphabetically at the end of the report. 4.- SI units A physical quantity is a physical property of a body or phenomenon, which can be measured quantitatively. The height of people can be known perfectly, so it is a physical quantity. On the other hand, beauty cannot be measured so easily, so it is not a physical quantity. A unit is a value of a certain physical quantity, which is chosen by convention and used to fix the value of different quantities of the same kind. Measuring a physical quantity is comparing its value with the proper value of the unit. For instance, you can use the length of your hand to measure another length. In this case your unit is your own hand and the value of the length is the number of times you have to place your hand from one point of a body to the opposite one. It is very important to share the same units all over the world, so that people can trade, sell and buy products fairly. In the XIX th century the General Conference on Weights and Measures established the Metric System, based on 3 units (metres, kilograms and seconds) and powers of ten. The International System of units (SI) was created in 1960 and is used by almost all countries, except the United States that still use the Imperial Units. Experiment 2: Try to measure the length of your table using a ruler (cm), your foot (feet) and your thumb (inches). Compare your results by pairs. Have you got the same values? Why not?

5 There are two different kinds of physical quantities: fundamental and derived. On the one hand, a fundamental quantity is easy to be measured and its unit can be defined universally in terms of a physical constant. There are seven fundamental quantities. Length is a quantity that measures the distance between two points. Its unit is the metre (meter, US). Mass is related to the amount of matter of a system or body. It is measured in kilograms. Time measures the duration between two events from the past to the present. Temperature quantifies how cold or hot a body is. It is measured in Kelvin. The amount of substance measures the number of particles (atoms or molecules) of a body. It is measured in moles. Electric current measures the flow of electric charges through a conductor and its unit is called amperes. Finally, the luminous intensity measures intensity of light in candelas On the other hand, derived quantities can be set in terms of the fundamental ones, so we don t need to define a specific unit for them. For instance, length is a fundamental quantity, which can be measured very easily, but area is a derived quantity because we can find its value with a formula. You can see some examples: Derived quantity Definition SI unit Other units Area L x L m 2 Area (100 m 2 ), acre, fanega Speed L / T m.s -1 Km.h -1, miles per hour, knot Density M / L 3 Kg.m -3 g.cm -3, gram per litre Multiples and submultiples Prefix Symbol Value Tera T Giga G 10 9 Mega M 10 6 Kilo k 10 3 Hecto h 10 2 Deca da 10 1 Deci d 10-1 Centi c 10-2 Mili m 10-3 Micro µ 10-6 Nano n 10-9 Pico p Sometimes we get a physical quantity measured with a certain unit and we need to change it to an International System unit. A conversion factor is a fraction which has the same amount at its numerator and denominator, but they are measured in different units. Conversion factors let us change a physical quantity to a different unit of measure without changing its amount. Imagine you have an area measurement, 2,3 harea, and you need to use SI units (m 2 ). As the value of a conversion factors equals to 1, if you multiply a value of a measurement by the conversion factor you don t change its amount. This ratio has both units in different sides of the

6 fraction: the old unit you want to remove is placed at the opposite side and the new one at the free side so as to simplify the old unit and leave the new one there m 2.3harea = 2.3harea. = 23000m 1harea 2 The old unit goes to the other side of the conversion factor The new unit goes to the free side The old units are removed and the new one appears alone. Activity 11: Change these units to the different ones in brackets a. 55 cm 3 (m 3 ) b. 120 m 2 (cm 2 ) c. 15 m/s (km/h) d. 3,6 g/cm 3 (kg/m 3 ) e. 300 m 3 /h (dm 3 /s) f. 700 t/h (kg/s) g. 70 miles/h (m/s) h. 12 t/m 3 (kg/cm 3 ) 5.- Error and accuracy Despite of measuring physical quantities with the highest accuracy, it is impossible to get an exact value of the quantity: every measurement has a certain extent of uncertainty. We just can reduce this uncertainty as much as possible and find the error of the measurement. Errors can be classified in two different kinds according to its origin: systematic and random errors. Systematic error is an error which always occurs at the same direction. Sometimes they have to do with the size or origin of a sample: if we ask people from Pozuelo de Alarcón about their income, the sample is biased because this city is one of the richest cities in Spain, so the information is always higher than the standard income of the whole Spain. Systematic error can be detected changing the experimental methodology. On the other hand, random error has the same probability of being greater or lower and can be reduced increasing the number of measurements. The absolute error is the difference between the measurement of a physical quantity and its actual value. As we cannot know the real value in advance, we take the average of the measurement as the actual value. Relative error is the ratio of absolute error to the actual value of the physical quantity. It is usually expressed as a percentage. Range of a measurement is the difference between the highest and the lowest measurement. As the precision of a measurement increases, the range decreases, so it is easy to use range of a measurement to find the uncertainty. ε = range 2 Significant figures are the digits we know without error. The number of significant figures of a value depends on the uncertainty of the measurement.