Transcript Document

Language and understanding in Physics
Acknowledgements:
Brian McKittrick, Kim Falloon
Helen McDonald & Geoff Davies
Force
• Forces are pushes and pulls
• 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
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
• 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
Forces and objects
How many vertical forces act on objects A, B
and C respectively?
A: 1 kg
B: 2 kg
C: 3 kg
table
earth
On A
On B
On C
By A
_
10 N
down
0N
By B
10 N up
_
30 N
down
By C
0N
30 N up
_
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?
Electricity – charge and motion
• The mean speed of a free electrons in thermal equilibrium is given by
𝑣 =
8𝑘𝑇
. In a room temperature
𝜋𝑚
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
∆𝐸
charge – 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 around a closed loop equalling zero.
Fields
• What is a field and 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 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 resolve G. I. Taylor’s experiment circa 1900’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 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 is the difference in the meaning of the words
“unbalanced” and “net”?
Student explanations and descriptions
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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 cell supplied energy to the charge as it passed through the cell
I can see my feet standing in front of the mirror
The electricity moved around the circuit at the speed of light
task instruction
Term
Definition
Typical learning examples
Analyse
Identify components, elements,
constituent parts of the whole and
identify the relationships between them
Consider presented information and clarify concepts
and knowledge; use qualitative and quantitative
methods to distinguish between components (words,
tables, labelled diagrams, calculations, graphs);
recognise patterns; identify and relate implications;
graphical analysis
Apply
Use knowledge (ideas, formulae,
Propose a solution or response to a problem or issue;
principles, theories, laws, models,
show steps; use algebraic and/or graphical methods as
techniques) in a new situation or context appropriate
Calculate
Use mathematical formulae and
Solve numerical problems by using formulae and
modelling to solve quantitative problems mathematical processes; find the numerical value of
an unknown variable or constant
Compare
Identify the similarities and differences
between two or more objects or
processes
List, tabulate or use a graphic organizer to identify
similarities and differences
Describe
Communicate the characteristics and
features of an event, object, procedure
or process
Use written or visual representations to communicate
characteristics or features
Design
Create a plan, object, model, system,
simulation or set of procedures to suit a
particular purpose ;
Combine knowledge, skills, materials and processes
to develop a solution to a problem
Determine
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 part; select from a number of
possibilities; select relevant information or
aspects of key ideas
Recognise and name/label a specific object, element,
component or underlying principle or concept;
label/annotate components of a system, model or
diagram
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 Construct a visual, physical, algebraic or graphical
principles; physical structures and systems representation of concepts, principles or processes
Evaluate
References
• Physics questions without numbers, Dick Gunstone and Richard
White, Faculty of Education, Monash University, 2012
• Physics 1 and 2, Halliday and Resnick, Wiley 1966
• http://en.wikipedia.org/wiki/Classical_field_theory