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The word ‘Force’, in common everyday, use has a much
broader meaning than the way we use it in physics. This point
is explored in download 1 ‘Lead lecture’ which is a
PowerPoint used to help Primary ITE students reflect on their
own understanding of ‘force’ and introduce them to some of
the conceptual barriers which students encounter as they begin
to form a scientific (Newtonian) understanding of ‘Force’
Driver R, Guesne E & Tiberghien A (Eds) 1985 Children's
Ideas in Science Buckingham: Open UP has two chapters
which explore problems children have in coming to terms with
forces and gravity: Gunstone and Watts “Force and Motion”
and Nussbaum “The Earth as a cosmic body”.
This brief article from Physics Education followed an ASE
Annual Meeting talk, suggesting that y12 students should try
out the elicitation questions about force and motion on their
family and younger pupils in order to clarify their own
misconceptions - a message we must pass on to our own ITE
students.
But more influential than just sharing research
findings with year 12 students might be to get students to
actually carry out their own mini research project using
similar simple free body diagrams. They might gather data
from friends and family but also from younger pupils in
lower years and so start to recognise and reflect on the
origin of their own misunderstandings.
Kibble, R. and Parker, B. (1997) 'Forces and Motion through
the Key Stages', Phys. Educ. 32, accessed from http://www.iop.org/EJ/abstract/0031-9120/32/2/002
in Feb 2005. This is also available as download 1.2
Type "misconceptions about forces" into a search
engine and you will come up with close on half a million hits.
Of course most of these are using the word ‘force’ as an
everyday way. Some of these hits, however, will link to pages
where the ideas are laid out in a teaching structure, and
student teachers who themselves are unclear about force and
motion should work through some of these. A good example
is at http://www.glenbrook.k12.il.us/gbssci/phys/Class/newtlaws/u2l3e.html
'Force'
is one of the 5 ‘key ideas’ of the KS3 strategy. The
rest of this section is a brief summary of the field. Many of
the issues below are addressed in the PowerPoint in download
1.1. You will also find the discussions in unit Professional
Issues/Teaching: teaching/Misconceptions, esp. download 6.2
When you give the trolley a ‘push’ you are giving it
kinetic energy. It is not surprising that people say things
have a force travelling with them, and that the object ‘runs
out of force’. If ‘force’ means ‘energy’ they are
right -rather than ‘runs out’ we’d prefer ‘the energy
is dissipated, through friction etc. as waste heat.’
Children think of forces in terms of movement, not staying
still. Children are likely to believe that if something is not
moving there are no forces acting on it. (Think of a person
trying to push a car with the handbrake on. The car does not
move so to children no force is evident. Yet scientifically we
know there are several forces at work).
The legacy from Galileo and Newton is that forces change
motion. Apply a force and the object will change speed or
direction. But apply no force and the object stays moving at
constant speed (which might be zero) and direction. No matter
how many times we refer to Newton’s First Law, pupils will
insist that moving objects have a force driving them
and they stop when this ‘runs out’. They cannot see that
the object slows due to the force of friction and that without
this friction the object would continue forever. The problem
is that there are no common examples relevant to them where
there is no friction. When we push a trolley we need a driving
force to keep going, only because of friction. So the word force
becomes used by most of us to mean something like momentum.
Momentum is a measure of how difficult an object is to
change direction or speed. The higher its speed and the
greater its mass the greater its momentum. Apply a force and
the momentum will change.
Cars, football and skidding Ask a pupil to run and
stop. Talk about the need to get a grip. Talk about icy roads,
wet floors and football boot studs. If a footballer, or anyone
else, needs to change their motion they need to get a good
grip. It is only when you attempt to change your motion and
can’t get that grip, that skidding happens. People often
associate skidding, or slipping, with turning corners, and it
is true that it is during cornering (or trying to start or
stop) that cars and footballers skid. But a skid happens when
there is reduced frictional force from the ground, so your
attempt to slow down or turn a corner fails, and you carry on
in the same direction that you were going. With no frictional
force (or grip) there can be no change in motion.
Children find the force arrows in diagrams difficult and
think they do not make sense. This is because scientists use
force arrows from the centre of gravity not the top or bottom
of an object. So there is no distinction between pushes and
pulls.
The PowerPoint in download 1.1 discusses the confusions
between free fall and terminal velocity in slides 10-13. ‘Weightlessness’
is another confusing idea. Most media reports talk of ‘zero
gravity’ to describe the effect of weightlessness in orbit.
But gravity is certainly acting, keeping all the objects,
including the space vehicle, in orbit round the Earth. We only
feel gravity when we oppose it - such as standing in the
ground. If we allow the gravitational force between us and the
earth to accelerate us, we will be in free fall, we
will feel weightless, and we call it, ironically ‘zero
gravity’. At KS5 we will need to look at Einstein’s
theory of gravity but Newton works well enough to KS4.
Children think that heavy objects sink and light ones
float. There is a grain of truth here, because, in everyday
language, ‘heavy’ can mean ‘dense’, and light can mean
low density - as in “Polystyrene is light, stone is heavy”.
Air is made of atoms, which have mass, so our atmosphere is
‘heavy’ and is attracted to the Earth by gravity. Why is
it, then, that hot air and helium move away from the centre of
the Earth - i.e. why do they rise? Accounts of the weather
frequently describe convection currents as being caused by ‘hot
air rising allowing cold air to rush in’. What they should
say is that the cold, dense air moves in, buoying (pushing)
the less dense hot air away from the earth against gravity.
Gases are real, massive forms of matter and we need to
appreciate that if they rise, something denser must be pushing
them upwards, just like water buoys up ice. Often no
connection is made between the idea of floating in water and
floating in air. Children need to place material half way down
in water to test them - if they move upwards they ‘float’
and if they move downwards they ‘sink’
Most work on forces should be done by the children moving
their bodies, or using their muscles - wonderful opportunities
to link the effects of forces on movement when children are
playing games in the hall or outside. We need to provide ITE
workshop sessions where student teachers can try out these
activities.
Water play can be just that - but if pupils are to begin to
gain some understanding of floating and sinking their teachers
need to see how this play can be linked to ideas. Controlling
the variables is the key: release objects halfway down under
water and change only one variable at a time:
- same shape (ball, cube) and size, but made of
different material.
- same material and shape but different size (and
mass),
- and finally using film canisters which have a fixed
volume and shape but can be made more or less massive
by adding air, sand and water.
Ice balloons make wonderful ‘playthings’ too.
These are some of the experiences trainee teachers need to
have in their ITE programme. Many of these are best carried
out in a school setting, often during the ‘practical try-out
time’ when trainees can try things out ahead of the lesson.
- An air track or puck table are essential for showing
frictionless motion, backed up by computer simulation
and video clips of space flight.
- Evacuating a tube containing a feather to show free
fall when no air is present.
There are plenty of more complex apparatus for supporting
student’s work on forces, such as ticker timers and dynamics
trolleys, but until the pupils are able to think in Newtonian
ways, much of this later teaching will fail.
Downloads in this document:
This Section prepared by:
Keith Ross, University of Gloucestershire
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