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Language and understanding in Physics Acknowledgements: Brian McKittrick, Kim Falloon Helen McDonald & Geoff Davies Student explanations and descriptions • • • • • The momentum of the car caused the brick wall to break The passenger flew through the windscreen because of the accident When the starters gun went off, I pushed myself forward The force of the moving ball propelled it through the air The cup is stationary on the table due to the action of gravity and the equal and opposite reaction force due to the table • The cup is stationary because the table is in the way Force • • • • • • • • Forces are pushes and pulls Pulls are attractive forces Pushes are repulsive forces Forces are created in pairs – a pair of attractions, a pair of repulsions; one force acts on one body the other force acts on the other body Forces cause objects to speed up, slow down, change direction, remain stationary and change shape Many forces can act on an object simultaneously Forces acting on an object can be added together – the net force as a mathematical quantity, not a physical quantity The net force is identical in size to the product mass of object and acceleration of object ∶ 𝐹 = 𝑚𝑎 or better still: 𝑎 = 𝐹 𝑚 Force 𝑑𝑝 𝑑𝐾𝐸 • The net force acting on an object = 𝑑𝑡 = 𝑑𝑥 , that is forces are associated with momentum and kinetic energy changes, which when combined with conservation rules lead to transfers and transformations of these commodities, a force or collection of forces acting on an object is now seen in terms of other dynamical quantities namely momentum and kinetic energy • In contemporary physics force is not a significant parameter, but rather an outcome of statistical averaging of field interactions. • Force as a word used to describe the transfer of particles associated with a field, such as photons for electromagnetic interactions or W-bosons for the weak nuclear force or gluons for the strong nuclear force. Forces and objects • A force acts on one object due to another object. • For example the weight of an apple is the gravitational force acting on the apple due to the earth • The table pushing up on the cup is the force on the cup by the table. This force is electrical by nature. The earth pulling down on the cup is gravitational in nature Force on cup by table Force on cup by earth Forces and objects How many vertical forces act on objects A, B and C respectively? What is the size and direction of all identified forces? A: 1 kg On A On B On C By A _ 10 N down 0N By B 10 N up _ 35 N down By C 0N 35 N up _ B: 2.5 kg C: 3 kg table earth Forces and objects • Object B moving to the right at a constant speed is pulled to the right by a horizontal force • Object B accelerates to the right is pulled to the right by a horizontal force mass • Mass is a measure of the amount of “stuff” a body is made of • Mass describes the amount of inertia an object has and hence the kinematic response an object has to a net force • Mass describes the strength of a gravitational field around an object • Mass describes the energy content of an object • Why does inertial mass yield the same numerical value as gravitational mass? • What might happen if there were mass with a negative signature and that momentum p = 𝑚 × v ? • Maybe equal amounts being created at the start of this current universe? Is space/time the real antimatter? weight • The weight of an object is the force of gravity acting on it • Does an apple weigh as much as the earth? • What is meant by apparent weight? Student explanations and descriptions • • • • The voltage passed through the globe causing it to glow The globe used up all the current The cell supplied energy to the charge as it passed through the cell The electricity moved around the circuit at the speed of light potential difference, current • Is potential difference the most difficult physics concept in secondary schools? • The potential difference between two points in a circuit, for example the potential difference across the LED • The current in the LED • The resistance of the LED • The power absorbed by the LED Electricity – charge, motion and potential • The mean speed of a free electrons in thermal equilibrium is given by 𝑣 = 8𝑘𝑇 . 𝜋𝑚 In a room temperature conductor, free electrons have a mean speed of 1.1 × 105 m s-1. The drift velocity of an typical electric current is of order 10-2 m s-1; this differs by a factor of 107. How do we talk of an electric current in this light? • The mean speed due to thermal motion results in electrons having a mean kinetic energy of 0.03 eV at room temperature. • Potential difference – the energy gained or lost per coulomb of charge ∆𝐸 – an open ended discussion about ∆𝑉 = 𝑞 • • • • Does charge have to pass through a cell to gain potential energy? General circuit theorems – conservation of charge leading to current conservation at a node conservation of energy transfer leading sum of potential difference around a closed loop equalling zero. Fields • What is a field? A field is a physical quantity that has a value for each point in space and time. • How is it used in secondary school physics? • http://en.wikipedia.org/wiki/Classical_field_theory • What are examples of fields? In what sense are they real? • Can a field store potential energy? How does this description stand with the language used to describe lifting a mass in a gravitational field? light • What language do we use when we describe reflection and refraction? • What language do we use when describing diffraction and interference? • How do we deal with the particle model for light – the photon – for some interactions and retain a wave model of light for propagation? • How do we talk about the photoelectric effect? • How do we resolve G. I. Taylor’s experiment circa 1920’s? • Is there a consistent language for use in modern physics? • What would a candle look like to a group of observers if it emitted electromagnetic radiation at a rate of 1 photon per second? inside the nucleus • What is the nucleus composed of? • Is it reasonable to think of the nucleus as being composed in individual protons and neutrons? • Both the proton and the neutron are modelled as being composed of 3 quarks. • A free neutron has a half-life against beta decay of about 10 minutes. energy, kinetic and potential An unbalanced force component acting on an object over a displacement gives rise to a change in the kinetic energy of the object. momentum A net force acting on an object over a period of time gives rise to a change in the momentum of the object. What, if any, is the difference in the meaning of the words “unbalanced” and “net”? task instruction Term Definition Typical learning examples Analyse Identify components, elements, Consider presented information and clarify concepts and constituent parts of the whole and identify knowledge; use qualitative and quantitative methods to the relationships between them distinguish between components (words, tables, labelled diagrams, calculations, graphs); recognise patterns; identify and relate implications; graphical analysis Apply Use knowledge (ideas, formulae, principles, theories, laws, models, techniques) in a new situation or context Calculate Use mathematical formulae and modelling Solve numerical problems by using formulae and to solve quantitative problems mathematical processes; find the numerical value of an unknown variable or constant Compare Identify the similarities and differences List, tabulate or use a graphic organizer to identify between two or more objects or processes similarities and differences Describe Communicate the characteristics and features of an event, object, procedure or process Propose a solution or response to a problem or issue; show steps; use algebraic and/or graphical methods as appropriate Use written or visual representations to communicate characteristics or features Term Determine Definition Typical learning examples Find out, based on reasoning, observations and information Make reasoned judgments or decisions on given or collected information, based on established criteria Recognise a quantity (often without the use of calculations); interpolate; extrapolate; estimate Assess the merit (strengths and limitations) of ideas, processes or procedures and reach a conclusion; validate evidence; choose from options based on reasoned arguments Explain Make clear; account for the reason for something or the relationship between cause and effect; state why and/or how Provide reasons mechanisms and outcomes, incorporate quantitative data as appropriate Identify Recognise particular elements of a whole or Recognise and name/label a specific object, element, part; select from a number of possibilities; component or underlying principle or concept; select relevant information or aspects of key label/annotate components of a system, model or diagram ideas Interpret Take a form of information and make conceptual meaning from it Derive meaning from information presented in multimodal texts (for example, written, aural and diagrammatic), tables, images and graphical formats Model Reproduce conceptual understandings and principles; physical structures and systems Construct a visual, physical, algebraic or graphical representation of concepts, principles or processes Evaluate References • Physics questions without numbers, Dick Gunstone and Richard White, Faculty of Education, Monash University, 2012 • Understanding and developing Science Teachers’ Pedagogical Content Knowledge; John Loughran, Amanda Berry and Pamela Mulhall • Physics 1 and 2, Halliday and Resnick, Wiley 1966 • http://en.wikipedia.org/wiki/Classical_field_theory